why could early 360s run with 250 plus HP stock?

Not sure how early you are talking about.My 73 had dished pistons.A friend had a set from a 71 engine that were flat tops.Everything else(?) was the same...maybe camshaft was different.Installed a Crane 260 cam(427/454 lift)and the truck would hit 80mph at the 2-3 shift whereas the stock cam would top out at 79mph in the 1/4 mile.What are you putting together?Bob