4 speed strength

i think there are so many factors its hard to say. A lighter car with more rearend gear and a good clutch even with 500hp will last longer than one with the same or less hp but heavier and with not so good clutch or steep rearend gear.

I think a 23 spline would be fine behind a 400hp smallblock no problems...but with the setup you have in mind for the future, id atleast step up to an 18 spline with a good clutch..Mind you though, theres no fun in driving a stickshift if you arent dumping the clutch and powershifting...mine as well just buy an auto imo

the interesting thing to consider though is that before the nash boxes came out, the mopar 833 18 splines and ford toploaders were the standard in 10/9 sec cars...keep in mind though like outsider mentioned that you will need to regularly maintain them to keep them running at those levels, and you will eventually break stuff too...its all in the clutch!

good luck!