For myself it comes down to reliablity, driveabilty and where can I go from here. For me the SBC 377 I am putting in will give me about 450 HP and be very driveable and reliable. If (when!) I need to bump up the HP, I know I can do it with what I already have and minimal additional expense. IMHO I wouldn't think you would get 500hp out of a 4 cyl reliably or end up with a very streetable car. But that is what I would consider reliable and streetable! I am sure some would feel differently!! But I like that I can take the SBC a long way before it gets to be too much of a PITA.