Someone selling a set of gen VI heads off a 2000 502 Mag in swap shop. Someone said that those heads wont work on a Gen IV block. Is this right? Theyre 088 square port heads and I didnt think there was a difference in the gen IV , V or VI heads. I know the blocks are different, thought the heads were all the same....please help.

some have said that with the right gaskets they will work..others say they leak...I would try to find a set of early rect port heads or aftermarket iron heads out there..thay may be cheaper in the long run after having to work the used ones..

they bolt on with no problems . i recently put a set of hp 500 gen6 heads on a gen 4 with o issues

I was told the gaskets too. It's explained in the GM Goodwrench Catalog in the cylinder head section. If I can find it I'll quote for you.

Yes the gen IV,V,and VI heads are different in the water transfer holes to match their corresponding blocks. Use of the wrong gasket will let coolant leak in to lifter valley. Mark VI gasket with a Mark VI head and so on.
The 088 casting can be used on a Mark IV block with Mark VI head-gasket, however the 188 casting (LS6) can't be used on the Mark V production block the water passages don't line up, but can be used on the Mark V tall deck and Mark VI block. These blocks were changed and can use all the heads.

Thank you for clearing this up.