New Clutch.....Problems

I'll bet it's the clutch fork.
A-Bodies take a 12 1/2'' fork and most others take a 10 7/8'' fork measured in overall length.
If not that, take a look at the bell crank (or z bar) to see if it's been modified.
If it was an ex race car, there's no knowing what they did to modify the linkage and or mounting points.
Check out brewer's performance website, there are lots of specs and pictures on there...........