Which heads wpuld be better?

I believe they ran till '74 or 75. The letters are cast next to the spark plug holes. Check E-bay (Not a fav spot) and swap meets for the letter heads. If your lucky enuff to have a wrecking yard with old cars, your probably in luck.
A search at randys ring and pinion, summit or jegs can turn up a list of ratios.
Don't stop me from you running 4.56 gears. It sounded so familuar (sp) I had to look into the MoPar Performance catolog to satify myself. The 318 12 second package in the tips section suggested 4.56's.