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

Not sure what the question is. But, if you took that same 8.5:1 engine and built it with a zero deck you would gain a significant amount of horsepower. The compression ratio with zero deck this could easily be calculated. I built two zero deck 360 magnum engines and they ended up in the 11.5:1 range, but the heads have a smaller combustion chamber than the old 360's

The 250HP are you refering to the 360 4bbl engine used around 1974? Why would that be a surprise that it made 250hp? It had a 4bbl carb, 340 cam, reasonably good exhaust manifolds, 340 valve springs, reasonable flowing heads etc.