Gen IV heads on a Mark 6 block?

[klmken]

Does this work? I know the Mark 5 heads dont fit a Gen IV block......

[BRUCE SEROFF]

If I have a gen 6 motor with merlin aluminum heads, can i use those same heads on a gen 4 block? if not , why

[Pat McPherson]

Gen. V and VI are the same. Gen. IV engines have vary different water passages.
Most after market heads are made to fit both with the right head gasket.
Gen V and Gen VI GM heads can not be used on Gen IV block. and....
Gen. IV GM heads can not be used on Gen V and VI blocks.