tranny q/a

I am the third owner of a 67 mustag with a built 302 it is scary fast. I just bought this car from my brother (2nd owner). The reason why he sold it was because one day he put it in park and it never shifted into drive again. I have park reverse and neutral. my plan is to have the tranny rebuilt. it is a c-4 and I was wondering would I be shooting myself in the foot buy having a C-4 built or should I just have a C-6 built. my concern is how much power can a c-4 hold up to? this will be my daily driver and I do want to hit the track occasionally. and the use of my stock shifter is a must. the look of a sleeper is what I want. Thank you in advance.