best size 2bbl carb or small 4 bbl for my 318?

A decent spreadbore carb will do it. Mechanical or vacuum secondaries. 2bbl carb bores are generally bigger than 4bbl primaries to handle power needs through the RPM range of the engine. When I had to drive my '68 GTX during the first "gas crisis" in the early 1970s, I put a stiffer spring in the vacuum can for the secondaries. Since they had dropped the national speed limit down to 55mph, I planned to drive at 60mph on the interstate to better mark time/progress. The stiffer spring held the secondaries closed to about 65mph. It worked great. It is a Holley 750.

I noticed some 6 cylinder 2bbl carbs with bigger bores than 4bbl carbs on many 1970s V8 engines.