[truckster]

A 1-piece rear main seal requires a different flywheel or flexplate (different center diameter) and that will be balanced for the 1-piece seal crankshaft - it has a weight on it and it's indexed with a pin so you can't put it on wrong. It's really no big deal.

I can't imagine why you'd want to go with a flat tappet cam when you can get the L31 Vortec with a roller cam and better heads for less money - by the time you add the intake manifold and fuel pump you'll still be money ahead. Read through the forums and you'll find a lot of people who complain about wiped cams, but not a single one with a roller cam. Plus the roller generally has a better profile because it opens and closes quicker.

You can get adapters for perimeter bolt valve covers for Vortec heads, but I like having valve covers that don't leak.