Sixpack on a 318 engine ?

There should be no reason you can't put a 6 pack on a decent (hp) 318, there's only a 12-22 cid difference depending on overbore over a stock 340 and a stock 340 was a pretty mild and relatively small engine anyways, No reason a 300-400+ hp 318 can't handle it, I'd bet a similar built 273 could also.

It more about the power of the engine (hp=airflow=cid x rpm) then displacement.