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.