I've read and seen pics where the old Mk IV heads have different coolant passages from head to block.The Mk IV heads have a round hole and the V&VI heads have a hole kind of shaped like a distorted L,that's where the mismatch and leak comes in.However there is a company(don't remember the name) that makes a gasket for this.This means the old 049 & 781 can be used on a V & VI block.Old hot rodders know these two heads are the ones to use when wanting more hp.I've often thought about putting a set of these heads,a good 4bbl Edelbrock intake manifold,and a TBI adapter along with my TBI unit to see what would happen.The only thing holding me back would be not knowing if the TBI injectors could supply enough fuel to make it run right.