I confess this is not a Cougar question per se, but it could be applicable. So far I’ve gone through quite a list with the T-Bird and have it running. It actually runs really well. But the question came up as to whether or not it’s a good idea to continue running it without replacing the valve springs. I’m not really concerned about the 52K miles, just the fact that they sat since 1979 with some of them at least partially compressed.
I would be more concerned with the timing chain and the valve seals and the fuel pump and any fuel hoses made of 50 year old rubber.
Ford used a nylon coated aluminum timing gear on 390’s and 428’s in the 1960’s. Those little plastic teeth come off and if the timing chain does not skip teeth first the little plastic bits stop the oil pump and shear the distributor drive gear roll pin.
New fuel pump and all new fuel lines. Valve seals are on the to do list.
Good point on the timing gear. I’ll definitely put that on the list.
What year Thunderbird are you working on? I love big old highway cruisers.
IIRC it’s a Bullet Bird.
I had a similar car/mileage/history of sitting and it broke a rocker arm and valve spring within 200 miles of driving after the start-up.
Asked and answered. 1963. Bought it mainly for it’s originality. Has had some minor repair, but it’s wearing mostly original paint and all original sheet metal. Got parked in 1979 with 52K on the clock. The interior is an 8/10. Pretty remarkable how nice it is for 53 years old. Born the same year as me and I doubt my interior looks that good.
So I’m guessing based on your experience, you would be in the change them out camp.
Born the same year as me and I doubt my interior looks that good
LOL. I hear you on that one.
Nah, I’m in the “wait and see” camp…it wasn’t the end of the world when that happened. I got a rocker assembly off a guy from the FE Forum and a tool from another guy to compress and replace the springs in the car. Bought the set of springs and seals and it took me a couple of hours maybe to do the job–no sweat. One thing I never do when getting an old car going, tho, is to venture too far from home! Daily drive it for awhile and see what starts leaking and rattling and what fails before going more than about 20 miles away. 
I’ll be having a similar issue with my '67 289 Cougar engine. One day I’ll transplant it into my '66 Mustang, not necessarily because it’s the engine I want, it just happens to be handy sitting there. I will need to know what needs to be done to it when that time comes.
The original Mustang engine also sat out of the car for 30 years. The thing I found when I took the heads off it was that there was very little torque left in the head bolts. I don’t know if it’s common to lose torque after sitting for years.
These 2 289’s also had the nylon teeth on the timing chain gear. They both fell apart back in the early '70s and were changed out for the all steel versions.
What about changing freeze plugs?
I bought a '56 bug in the UP that had been sitting for years. Fired it up and started driving. It broke a valve spring about 375 miles into the 425 mile trip home.
That’s excellent advice.
X’s 2 on this advice.
I think I have a crush on Kim, she’s awesome! Can I get you a beer while you overhaul that engine, honey? 