As the title says, I am wondering if there are 31 spline axle shafts for an 8in housing. My cougar has a 289 that with an Edelbrock top end kit should make ~300hp (kit is rated at 321hp on a 302). I am planning to have my cougar weigh around ~3000lbs when fully built. I am going to use it as a DD as well as use it for auto cross a few times a year. I will be converting the C4 auto to a T5 5 speed manual. If you think I am good sticking with 28 spline axle shafts I don’t mind doing that, I just want to go with what I need in the beginning and not have to buy a new third member and shafts again.
Probably easier and cheaper to swap in a 9" from a 1969 - 70 Cougar. Then you are probably fine with 28 spline axles. 31 spline axles are needed for drag strip starts. For auto cross 28 spline axles with some sort of limited slip differential would be fine. Choices are limited and prices high for the weaker 8" differential.
Right and left axles interchange for all 8 and 9 inch Cougar and Mustang 1967-70. Stay away from the very early 1967 axles that were tapered.
9 inch rears are stronger than 8 inch third members, but the 8 inchers are pretty stout. If you aren’t dumping the clutch to light up the wheels, the 8 inch with 28 splined axles should serve you well.
So the answer is yes, you can order 31 splined axles for your car. Select your rear axle ratio with the T-5 and tire size in mind. I have a 3.50 gear set in my 1968 XR7 with T-5 and 302. I selected my gear set so that I could cruise in 5th gear at 60 mph without lugging the engine.
Ah okay, didnt know early ones were tapered. I have an XR-7 which if I remember correctly were the 2nd half of the build year (I have a 1967), assume I should be good then? And yeah I have the T5 + tire size + rear end math all done so I know I have a good combo! 
Bear in mind that the 31 spline axles take different bearings and seals. Those parts from a '68 - 70 Mustang / Cougar should fit a 8" housing. Also to use 31 spline axles you will need a 31 spline differential that fits an 8" rear end.
The T5 will break before the 28 spline axles do.
Totally agree.
Good to know, thanks for the input all!
The 8" rear end is generally thought to be good up to 300 horsepower and 400lbs of torque.
One also has to remember that the 9" rear end is heavier is thought to produce more friction slightly reducing horsepower.
My 68 has an 8" & the only thing I would change would be to get some type of limited slip.
There would be no notable loss of power with the 9" differential. The benefits far outweigh any possible negatives. Not to mention the selection of gear and part offerings for the 9" it’s a wise upgrade if you’re considering any performance engine upgrade. 28 spline is fine. Even 390 Mustangs and Cougars came equipped with 28 spline axles.