These flights of "What If" thinking are interesting, but I prefer building more common stuff.
I understand that a free motor sitting over in the corner can eat away at your gray matter, but add up all the parts and machining.
What percentage would a 175 dollar You-Pick-It 360 magnum be? On a 1750 build (CHEAP), it's 10 percent. If your dream engine is 330 cubes, you are getting 30 extra cubes, almost 10 percent for that extra 10 percent in cost. We won't even talk about roller cam, better breathing ports, and less valve shrouding.
The first issue with RPMs seems to be piston speed. The second is valvetrain issues. Try to find the circa 1979 Car Craft issues that had Dyno Don's P/S Arrow in them.