I went to your car domain page, but I couldn't read your dyno sheet readout for your engine. Are you running a stick or an auto?
You'd have to have a pretty potent small block to hurt a 31 spline 9 inch with an auto. If you put 35 splines in when you don't need them, you are using HP to turn that stuff.
I'd stick with the 9 inch simply because swapping gears is easy if you don't hit your mark the first time. Plus 31 spline third members are relatively inexpensive. For my 56 chevy I have two third members, one with a 4.10 for fun and a 3.00 for power tour and stuff like that.
By the way give Doug at http://www.quickperformance.com/ a call he's a moser dealer. Tell him the HP you plan to run and he'll tell you what you need, he's areal straight shooter. You can get a bolt in 9 inch housing for $345 or the housing with axles, etc. for $715. Tell him Dan from Craftsman tools sent you.
Good luck.
DS