Hi Guys I been following this thread, from Oze, and Im doing a rebuild right at present with Gen VI 454, with MKIV heads.
what I have determined is the heads will go on the block, but which head gasket is used.

I have a few copper SCE, Felpro 8081T gaskets from MKIV style heads, when I lay the gaskets on Gen VI block and MKIV head, there appears to be not much difference, only the size of the water passage in head/block being different, same location.
In Gen VI block the 3 bottom (along block) water holes are larger, than the same MKIV Heads which have circular holes

unfortunately the GM gurus dont want you to this head swap, as not much printed information about it. Can anyone add to this thread