Can I use these heads on a early model block? Someone told me you cannot, but I don't see why not??

You CAN'T put a Gen V or VI head on a Mk IV block. You CAN put a MK IV head on a Gen V or VI block. 99 out of 100 times you try a Gen V or VI head on a MK IV block, they will leak water into the lifter valley.
Eddie

Thanks Eddie.