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

You couldnt mill a block far enough and keep enough meat in the deck to get factory flat tops to zero deck. The last '87 360 I tore down had a factory center dish, but basically a flat top type ring, and they were .180" down in the bore. The areas around bolt holes on the deck are thick, but there are spots in the deck that get as thin as .300, and you'd be cutting more than half off. Other factory flats are way down there too. Plus, it's extra meat for milling, and that means more $$. I had extra passes on a block cost me $200 before becuase it was "extra work". Better pistons are $280, and makes the intake fit right, and pushrods fit right, and bolt holes deep enough to clamp well, etc. It get's back to "is it worth it?" The answer is no. But, if you built it right, it's easy to get 275-300hp from a factory type piston and no compression or quench. But you have to get the right package and realize the limitations.