Yay, success today! I redid the basics, confirming compression TDC on #1, and I got really nit-picky about the rotor position. I repositioned the intermediate gear untii I was able to get the rotor to point exaclty at the #1 terminal, not a ballpark as before. I think the trouble is that with the intake I'm running (LD4B) the 6 and 8 runners offer too much interference to the vacuum advance, eating up a range of sweepability of the distributor. For my previous attempts, I'm guessing I happened to have the shaft keyed to a spot that does not allow the sweep to be in the range I needed it.
Thanks fellers!