It does the exact same thing at the exact same vehicle speed (like I said, no tach, I get rpm from the timing light when setup in the carport) with or without vacuum advance. I've tried several setups with the initial (18/36, 16/34, and 14/32) and get the exact same result at the exact same speed except that it likes to get warm with the low initial.

I think that it is something else at this point. I'm running Harland Sharp rockers and maybe I have too much side clearance on one pair (I tried setting them at 0.020 per set of rockers based on advice from a well respected individual here on Moparts), a bad lifter, or something like that. The sound to me is clearly from the passenger side of the engine (but I grew up on a farm around very loud equipment and have always liked loud engines and loud music and started losing some hearing a few years ago).