I am thinking about buying a 1990 25' Mirage with a 516 Big block chevy with a B&M 250 blower. Any advice on a 502 being bored out to a 516.

Just a .060 over 502 don't see a problem unless you mess up something from this point on....then you have a issue...

That block is only at 4.530. You can safely take it to 4.600. If it is a pristine, fresh water block and you continue to run in fresh water, or it is fresh water cooled, you can bore it to 4.625. Salt water eats them out from the outside in, that is why I say it needs to be a fresh water block to go to 4.625.
I can't even count how many I have bored to 4.600. I have never had a problem with a one of them.
Eddie