OK, so I just ordered 2 GM marine 502 longblocks (factory) from Derebery performance. They are new,factory GM built units. Do I need to do a teardown and tech inspection (verify proper clearances) Although these are supposedly marine standard engines, they dont have inconel ex valves. I thought that was one of the standards on marine engines. Any advice?

Tear them down and go through them. I have seen them be in and out of spec. You are there...they are not complete....take the time to triple check everything.

Do you really trust a factory massed produced engine??? It's your investment....it would be wise to have a machine shop who is successful and familiar with proper ring/bearing clearances/tolerances, thermal expansion, piston to cylinder wall, etc, etc.

I hope it all goes well for you.