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Chris deHahn contributed the following on Shovelhead heads.
TAKE THIS FWIW. It is not gospel, the absolute truth, or the American Way. It is based on my experience and those I trust. Corrections are encouraged.
Head castings were changed significantly during the life-span of the Shovelhead motor. The castings have the date cast into them. Basically there were three major revisions of the head casting. The first appeared on the generator case Shovel and lasted into the early seventies. It is distinguished by a row of equal height fins in the center, between the rocker box areas. These are decent heads, but since they're old, many aren't in good shape. Valve seats were much improved over the Panhead, made of a hardened steel alloy. However, if you find a set of these heads, be prepared to replace the seats, as most of the used ones you'll see at the swaps have been cut several times and may exhibit significant valve seat recession. Seat replacement cost is around $50/ea X 4.
The next revision came in the mid seventies, around 1974 or so and lasted until 1977 or 1978. These heads, known as crown heads, had a row of fins in the center that formed an arc, or crown. The ports were opened up significantly, yet they had a lot of extra meat in the casting. This is why they make a good base casting. There's a lot of potential in them.
The last revision was in 1978 or 1979. These had no fins in between the rocker boxes, and are known as late heads. The late heads had very little extra meat in them to remove for porting work. In 1981, they changed the spring deck so that adding even a mild aftermarket cam meant you had to cut the spring seat to gain enough clearance, or fit high lift spring collars.
The dates are approximate because AMF, in their infinite wisdom, used whatever castings were available at the time to assemble motors. It is not unusual to find a bone stock Shovelhead with one crown head and one early head on it.
The next time you're at a swap meet and find two different types of Shovelhead castings, compare them. Look into the ports. It's pretty easy to see which ones are the better heads. Crown heads are in demand by HD drag racers and command a premium. You won't see too many at swaps.