I wrote something about this earlier but erased it.  But now that I've thought about it more it might actually be a possibility.  Rotor phasing.  The lower initial timing might cause the rotor to spark on the terminal before or after the proper terminal.

Rotor phasing is weird because the the mechanical advance moves the shaft in relation to the pickup coil.  But the rotor is on top of the shaft so its phasing moves also.  centrifugal advance moves the phasing, vacuum advance does not because it moves the breaker plate.

I could probably work it out in my head but it's already taken some effort just to pull that out.  I remember spending some time trying to get the burn pattern more centered on my rotor back when i was messing with timing and high energy ignition.  It was hard to get more than hall of the rotor terminal burned though.  Not the center just one half of one side.  If less than one half of your rotor terminal shows a burn mark the phasing might be off and its sparking to the wrong terminal.  

A thought.

Hey Guys

I appreciate all of the thoughts and ideas.  I did take the distributor back down to look at the mech advance again.  The Springs were a bit mushy so I bent the tabs out a bit to tighten them up.  I think this should allow me to set the base timing at idle and hopefully have the entire slot used for rpm dependent advance.  Don't know if it worked yet because I went to start it back up and it didn't fire.  Did some checking and didn't have voltage at the coil or IM.  Checked Fusible Links and one is burnt.  That's as far as I got before dark tonight.  Will be at it some more this weekend and will update on progress.  I'm happy to be finding these issues and getting them resolved before making this my daily driver again.

Hey Guys

Thanks for all of your helpful comments.  I managed to get it going and producing power nicely up to redline.  I did end up replacing the module.  I can't be certain which fix was actually causing my problem.  I suspect that both conditions(mech advance springs & IM) were contributing but the IM was at greater fault.  I've never known a timing advance issues to have a discreet cutoff point and pop and crack in the manifold with the throttle wide open just sitting in the driveway.  Will take it in for smog tomorrow.