I am running a complete Mercruiser 165 in my 51 Chev , using the original head -- No apparent difference . Also the OFFY / tube headers / ps pump fit right on . Works great ! We cut the head .080 , 1:84 intakes , stock exh , unshrouded the valves , multi-angle grind , profiled the head bolt bosses and minor port matching -- Hope this helps -- DON

51 Fleetline / 250 /T-5