you have a 70to 90 mark 4 block and 91 mark 5-6 heads the deck is not the same on the older blocks so old heads on a new block they will not seal very good in the intake valley and you could have a leaker but the deck on the new mark 5-6 heads are wider were they seal up by the valley so they could work on the older block i have done it with the right head gasket but you need to check it over put the head on the block with out head gasket out line with a marker pen in the valley side of the head take the head off look at the out line if there a good bit of sealing behind you mark you will be ok