If you're worried get some 28 spline forged axles made up. 8" is lighter and already is installed. 31 spline change over will cost some $$$$. Unless you're making mega HP and slicks you won't have a problem.
If you're building from scratch with no axles or differential already in your parts-pile, I would definitely go with 31 spline and as already said -never really have to wonder about it again.
If you've got a salvage unit with 28 spline axles already in it, then you need to look more carefully at budget and the power & way the car is used... Look at the 'Project '67 / Project 11.99' car in FordMuscle's project cars to see what 8" and then 9" with 28 splines will or may hold up to.
No reason to pick the 31-splines unless you gonna put serious horsepower and slicks.
There is no way to twist of a 28 splines axle with regular tires and ln the medium HP range.
I have used all of Ford 8'' thrugh 9'' axles with different splines and there has never been any problems.
Even the weaker 8'' with 28 splines hold up for 400Hp without problems.
I never run with slicks though!
For 65 Mustang the smaler 9'' banjo with 28-splines will be easy to install.
Standard 8'' axle for 65 Mustang have a very thin tappered banjo.
Many of the 31-splines banjos are huge and wont fit without major machining and they are defenetly way to wide for the narrow 65 Mustangs body.
Why not buy a new 8'' 3-member for your stock 8'' from Currie? Will easy hold up for 4-500Hp
But you maybe has more serious power in mind?
<font size=-1>[ This Message was edited by: MrPer on 10/4/06 2:47am ]</font>
Got a friend that runs 6.80's 1/8 mile at 100+ MPH. Took him two years to break a 28 spline stock Ford axle. From looking at the failure, it developed a crack or had a fault in it already - it was not a clean break, it was actually a tear and twist as the metal came apart.