My friend John Young is building both an older 258 head for me for my '74 Gremlin ( porting, polishing, etc ), and a 4.0 head for his '66 Rambler hot rod convertible project. His email is : NOSIGMA@xxxx . Not quite sure how far along he is on the 4.0 head, but if anyone can give you direct experience/help on bolting up the 4.0 head to an earlier block, and give you some idea of the performance gains, he'd be able to. There are some differences in the gasketing/coolant flow, but it shouldn't be a difficult swap. He's even got his own flow bench to test the gains! He's posted results before on another Yahoo Groups list , called AMC-DelMarVa, so you might want check that forum's archives out for his head comparison results on the flow bench. Jerry in Virginia, 6 Gremlins parked in the driveway...