Mark v heads on Mark IV block?

[Rannos]

Am building a 496 and was going to use a set of 049 GM oval port heads. I came across a set of 088 rect. ports off a 502 mercruiser for a reasonable price. I have heard you can run the newer heads on an older block but not the other way around and I have heard you can use older heads on the newer block but not the other way around. Both versions I have heard you have to use a special heads gasket. Has anyone done this? If ya have do you have the part number for head gaskets the later model heads to mark IV block?