318's were the work horses. They just did not get the high performance goodies. Big port heads were found on 340's or any 360, 4 barrel intake and carb, performance cam and valve springs, double roller timing chain, quick curve distributor, good exhaust, and to make it live a high pressure oil pressure spring and a windage tray. It's pretty simple really, they will all just bolt onto a 318. As for bore size, a 340 has only .13 inches larger diameter and a 360 is only .09 bigger diameter. All things being the same, you'd be hard pressed to tell the difference between any of them.