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Matthew: Thanks for the info. Armed with your knowledge, I got a Cloyes 2764 gear set. Before I did so, i phoned them, and, after looking at the blueprint, assured me that this set is straight up. After I got it, I traced bore and the dotted tooth very carefully, and then put the old gear accurately on top of the tracing. I did three runs, just to make sure: got the same result every time. There are 29 teeth on the crank gear, so each tooth is 12.4 degrees. Sure enough, the new gear is advanced about a third of a tooth, so i can tell you with certainty that, in 1981, the smogged sixes had their cam timing retarded 4 degrees.
Interestingly, the original gear in the truck has 2764 stamped on it. The tooth dot; however, is not in the center of the tooth, but ahead a bit.
If returning the cam to straight up makes as much difference as it did in my 84 460, I will be happy indeed. Hope this info is useful to another shade tree mechanic out there. JB