Swapping out gen 5 454's for gen 6 502 blocks. Anyone know if the flywheel is the same or do I need to buy new ones?

Should be the same.

If they were the low hp 454MPI's with cast cranks then it's a different flywheel. The 454MAG uses the same forged crank and has the same flywheel as the 502. There is an extra counter weight cast into the cast cranks so it takes a slightly different flywheel. If they're the wrong ones, any good machine shop should be able to balance them to the GM502 specs for cheap.

Is there a # on the flywheel I should look for to know exactly what I am working with?

You need the flywheel with casting # 805. The 805 will be on one side and the other side will have an 8 digit # ending in 805.
Eddie

Thank you very much!!!

Well I guess 1 thing is going my way. 805 on the flywheels! So I guess these are what I need then.