When you have a 40 year old car that is what you expect. I have a 65 Malibu SS. I'm replacing 1/4s and floor pans. I bought the left side floor pan and replaced chunks as needed. Don't do that replace the whole section it is a lot easier in the long run and looks better.
A 327 came in your car originally. I replaced mine with a 96 LT1. The 396 is a lot heavier so new springs and front disk brakes are a no brain-er. The other issue is frame mounts are off so some brands of headers will not fit.
I started my rebuild from the frame up. Go before show!
Good luck! Remember this was the 2nd year for the Malibu and the number that are around is getting smaller not larger.