1965 Rear End Questions

[JWheeler331]

My 1965 C10 Short Wheel Base truck has the original rear end. I am planning down the road to put a stronger LT1350 in the truck once its done.

My questions are this;

I guess I will need to change the rear end to a posi rear end. Should I change the whole rear end or just the innards within my stock rear end?

Do I just need to put in a better gear and posi or is that something I would need a whole new rear end to get?