if im not mistaken the 3.3 is just a slightly modded .15. I dont see were you are saying that with an .18 or .21 you are loosing power or has less power than a 3.3. The only way i would suggest going to a big block .28 would be if you have the money to repair diffs and spur gears and tranny. but like others have agreed with is an O.S .18 that would be the bigger bang for your buck. 1. its cheaper than a .28 and the converson kit to get it to work 2. less maintenance cost. Like i said if you have the money to go big block then do it