318 all day. Took a 68 318 block and put it in 273-4 clothing (heads, induction, cam, exhaust...just the 318 short block) and it was more fun than the 273-4 ever was in my '65 S. The 273-4 would break the wheels, but the 318 would smoke them with a 3.23 SG. And that was without the compression bump the 273-4 had. It had 45 inches on it! Thats ~17% more motor.