Hi, Im trying to build a 1988-1992 "575 GM 540"

I think its a 502 Block Non Roller and 4.25 Crank and 088 Heads.

Would it be better to use a 502 Roller Block?

1- What Pistons and Rods?
2- Camshaft?

The only real gain going with a gen 6 would be the one piece rear main seal, but I've never had a problem with the two piece!! the lifter bores are higher but again don't see many problems with the older blocks.
I would defo go with the later step nose cam and use the gen 6 cam retainer Comp Cam 01-446-11 would be a good cam.

not sure what you are trying to build a 540 cu in motor??

a 540 is 4.25 stroke and 4.5 bore

i run isky red ball series solid roller lifters, 700 lift 285/295 duration

my motors a mark4

srp piston 10.3 to 1
eagle rods

never had issues with two parts rear main seal.