[markjohnson]

The biggest difference between those two heads is that the the exhaust valve diameters are larger on the #291 (1.88 Diam.) The #840 still had the 1.72 exhaust valves. Now somebody enlighten me as to what kind of seat is on the spark plug of the 1969 #840 heads. I thought that 1970 #291's used a short-reach, tapered seat. I've heard of several late-1969 L78 cars using a #291 cylinder head on one side and an #840 head on the other. That points towards the #840 head being phased out even before the 1969 model year was over so I would think that ALL 1970 L78 and LS6 engines used the #291's. Especially the late introduced 1970.5 Camaros.