I cant get live chat to work sorry.
Do you have a electronic tach hooked to the HEI?
Here is what id look at i have run into a break up on the HEI with a few different causes,
Remove the wire from the HEI for the tack,, then test for the break up,
I have seen a electronic tack do some weard stuff to the HEI
If were ok there, pull the cap, remove the rotor,,, check the counter weights
should move freely and snap shut with ease..
if were ok there don't put the cap on yet remove the distributer from the motor
check for end play on the distributer shaft not up and down but back and forth,If you can move it around its doing that spinning in the cap,,,,, will cause a miss and fluber
If were ok there check the other end see if it is spinning freely from by the end that goes into the motor, there is a brass bushing there can be bad
and last thing to look at have seen but not very much
check the power at the distributer at 3500, 4000 rpms should be at 12 volts
if under 11.5 the HEI is nor getting enough voltage,, could be ignition or regulator
One of these will get ya going