YEAH 35 spline is what you want. If you were going to put 14x32 bite on it and really best the piss out of it I'd say 40 spline but you'll be golden with 35.
I came up w/ a 72 vega built by east texas race cars back in 1994, finished it was a 600 horse sbc, turbo 400 . Had a ford 9" w/ 31 splines, it was fully tubbed by e,t,r., and I ran 14x32`s on it. The splines twisted enough that the axles were hard to remove and re install , but they never broke.
Wish I still had it !