People seem to like the 18. To get the full benefit from it you'll need port work, a good pipe, and a different intake. I would suggest a after market head as well. If you're making a hod rod you need something to help it run cool. They also give a little more power, and well they just look good.
Bigger engine needs bigger carburetor. Smaller engine CAN use bigger carburetor, if it's modded. It will only help at higher RPM's, but you'd need porting for that and a bigger intake so the air fuel mix actually has more space to go into the engine. But you can go too big as well. If you go too big you may not be able to get it to idle good.