It is a myth that twins spool faster than singles. Twins are generally better when you have less room to run tubing because there are no x-over tubes. 330c.i. is not a lot of displacement but with a properly matched exhaust A/R, will spool up quick. People will talk turbo lag, but if that's the case, there housing is not matched to there combo. Just as a example, I'm running a 76mm on a 6.0L, that's only about 30 more C.I.s, with a .96A/R exh. I'm also running a 3400 stall converter from a previous combo. which is a little to much. It achieves full spool at a little over 3000rpm. The trick is, you want your stall speed to be just before your turbos spool time on a street motor where your not in boost everytime you go half throttle to merge or pass because gas mileage and street ability will suffer. There is a bit of science to it but the outcome is just sick
76mm. is plenty for the street because it still has good spool time for that many C.I.s. Just depends on how much power you ultimately want to make. BEFORE you start, that's the main question you want to ask, along with do you want your power to come in low rpms for towing and mudding, or do you want a high revving eng. where it comes in later...? Don't just throw one on or you could end up like those stupid import guys that have to get on the interstate at 80+mph. and can't race a go cart from a dig at a red light.