Even with the 318, its probably a 3.21 since its a road runner, thats what one of my brothers 74 came with. Unless you can hook up with slicks, you will never hurt that 8 1/4 rear. I've done the bone stock 318 then cast intake 4 bbl with sq bore eddy carb/adaptor swap with headers and its makes for a quick reving, plenty of street power and great sounding combo on a budget. Nice looking car