I did a quick search to try and refresh my thinking and didn’t really see what I was looking for. I will check MC.net too. Anyway, as I recall, adding front and rear sway bars is always a pretty god plan, but it seems to me that I read that the rear must always be a smaller diameter. Is that correct? Even if not a lot smaller, it should be smaller. I have a 1inch in the front of my 69, and a year or so ago got a really good deal, on a rear bar that is also 1 inch. One of these days I will stop buying parts just because they are a good deal. Hopefully. Are equal diameter bars front and rear an acceptable combination?
A one inch bar on the rear is far too much for a 69 cougar unless you have extremely wide tyres on the rear (and narrow tyres on the front).
When deciding on what sway bars you need, you first must decide what the car in question needs.
There is no one solution fits all. Things like engine weight over the front wheels, body style, tyres size and type, suspension type, “A” arm position all affect the size of the sway bars needed.
A cougar with a 351 windsor is different than a cougar with a 390, A convertible is different to a coupe.
If you already have a 1 inch bar on the front, you next modification is the front top “A” arm drop, (shelby drop). Then go the rear bar and that size will depend on the handling of the car with the shelby drop.
A one inch on the rear is for serious racing only, you will not like it.
Peter 
Some times those good deal parts are cheap for a reason. I’ve got a couple of shelves full of them.
You own it already so throw it on and give it a whirl.
Generally speaking, rear anti-sway bars were smaller than front anti-sway bars as automakers produced cars that understeered (additional turning input did not produce additional turning) than cars that oversteered (light rear end/fishtail). They work by adding to the spring rate when turning but are of no effect when not loaded (going straight). While I agree with Peter that a cars configuration does impact anti-sway bar applications and subsequent bar diameter and “arm” length, I disagree that this would only be for racing. We don’t know your car configuration (well I don’t) nor how you drive or want the car to handle so again I say mount it up and give it a try working up to speed slowly and gauging the cars response to your inputs.
Glad to know I’m not the only one!
I was/ am thinking about my 69 for this part. It has a decently built engine, 3.89 gear and a T5 trans. Spirited street driving, or up and down the mountains near where I live. Estes Park is only about an hour or so from my house, and it is a nice twisted ride there and back. Lots of fun on the motorcycle. I have a stock 1/2 inch rear sway bar from a 70, and then saw the inch piece, complete with all the hardware, for 75 bucks. How could I pass that up? The car by far, spends most of its life in the garage, and I live in a mostly flat area. I was concerned about the rear end wanting to come around, which was my motivation for the question.
I have a Hellwig 3/4" anti-roll bar in the back and a factory big block (15/16") front bar in my 69 XR 7 that has a 351W/FMX. Polyurethane bushings on both bars. It handles really well, although it feels a bit tail happy compared to my last few daily drivers (a trio of Ford Fusions and a pair of Subaru Outbacks).
Several years ago I had a 72 Mercury Montego wagon. I put a rear bar from a Lincoln Mark IV in the wagon. It made a noticeable difference in that car’s handling. IIRC that bar was probably 5/8 or 11/16"
The "RestoMod’ has a 1 1/8 front & a Stambar adjustable rear bar. Ive also done the Shelby drop and custom bent upper A-arms (ball joint bind correction) Along with a special alignment, this Cat handles better than most cars that show up at open track events. If anybody want more info just ask.
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In my youth, I decided my '78 F150 needed to be sportier. Now Twin I-Beam Ford trucks aren’t known for their handling prowess, but I decided I could change that. I added some KYB shocks and the most aggressive sway bars in the Quickor Engineering catalog. Initially, I was thrilled with the results. Body roll was virtually eliminated, and it took corners like no pickup had a right to do.
Then I drove it in the rain. At the first medium sharp bend, the back end came suddenly around and the whole truck spun 360 degrees. It was a minor miracle I didn’t crash into anything. Next corner, the same thing happened, although I was anticipating it this time and was able to keep it to a major fishtail, rather than a full spin out. I had to drive slower than grandma to get home. After a few more close calls, I finally disconnected the rear sway bar which made the truck safe to drive again, although with the big front bar still in place, understeer was pronounced. A stock rear bar salvaged from a Camper Special truck (about half the diameter of the aftermarket one) was my eventual solution.
Anyhow, it taught me a lesson about oversize rear sway bars. On my '69 Cougar, after considerable trial and error, I’ve settled on a 15/16" front bar, adjustable 5/8" rear bar, factory “Competition Suspension” springs, 3-way adjustable conventional (not gas) shocks, roller spring perches and the Shelby/Arning drop. This is not the ultimate track setup by any means, but I find it gives great street handling while maintaining a comfortable, compliant ride.
I realize what works for one person may not work for another, so use the 1" rear bar if you want to. Just be careful in the rain!
Good story! I have a factory 70 rear sway bar,minus the hardware, and I have another front sway bar on the shelf. I’m not opposed to selling the one inch bar that I have- I don’t really know what else it would fit, or be used on. The 5/8 bar sounds more like what I want for a street car.
A 1" rear bar is way to big!
The factory rear bar is typically 1/2", which works well with a 15/16" front bar and OEM competition springs. I auto-crossed my big block Cougar with a 15/16" front and 5/8" rear bar and found it to have good balance.
However, with a 1" front bar, you will probably need an 11/16" or 3/4" rear bar.
Unfortunately Addco no longer makes the 3/4" bar but insists on selling a 7/8" bar. I ran that 7/8" rear bar along with an 1 1/8" front bar on my '67 SB road race car until I discovered it caused the inside rear tire to get light coming out of tight fast corners - making that tire loose traction and speed.
The geometry of the Cougar rear bar can be replicated with a simple U-bend, so replicating a Stam-Bar is possible (good since Stam-Bar is out of business). Constructing the sliding-block clamp used to connected the upright is slightly more difficult.
Just noticed that Cobra Automotive still lists 5/8", 11/16", and 3/4" rear “Stam-Bars”.
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Thanks very much for your insight on this!
So then, what would be a good application for this sway bar? Would it fit an F-100? Who could use this part? If I can’t use it, I’d like to find it a new home…
The 1 inch rear bar would be good for someone who wants to run lower rate springs for a cushier ride but more bar to prevent body roll, or for someone who runs a big tire (some are running 295 or 315 tucked in on a 69/70) on a serious handling car doing stuff like autocross where steering with the throttle in tight turns is the norm.
Someone doing auto-x with a 67-70 might love to have that just to have it in their sway bar spares arsenal.
Depending upon your price and location I could be interested in it woodsnake