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

look up dyno'd 360 combo's in the search engine of this web site, see what kind of power can really be made. You gotta be patient, bro, take the time to save for the right basic components, like good pistons, and the rest is exactly what has been implied...Bolt On. The amount of rated power in the 360 also changed because the method of measuring that power changed during the advent of the smog era. You might get a book like "Building High Performance Chrysler Engines" that give some of the history of the LA block and how it progressed through the years. It will answer all these questions, too.