Off the Mortec site...
10237297...454.......96-up...4-bolt, Vortec 7400", "L-29" Gen.VI
I do believe you can run Mark IV heads on a Gen VI block...it's the Gen V blocks that can't run 'em. The main thing to check is to make sure the water jacket holes in the heads and the block match up. But I do believe Gen VI will accept Mark IV heads.