Andy knows more than most, So I read his post and would tend to put a lot of weight in his words. With that said, enough blow by to push the dipstick out of the tube, coupled with RPM related oil leaks at the pan gasket, do not seem to be a good signs to me. Considering 500 cubes in a lowdeck generally means fairly short pistons, I'd be tempted to pull it and ask the builder to tear it down and take a real good look at the rings & skirts for evidence of anything unusual that could have happened upon initial start or break-in. ???.... your choice may vary.

Good luck! and keep us posted.