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

I think he's just sying a all stock 360 with zero deck pistons, then the other part, of course are changed, cam, intake, etc...

Sounds like a regular hot rod/performance build to me.

Why he would think this wouldn't last is beyond me. It's only done all the time. A normal performance build is just that.
A swap of compnets starting with pistons to desire compresion ratio with a upgrade in cam cam related compnets, intake and carb with headers etc....
normal stuff.