Harms Rebuilt '71 Thermoquad - Wondering Idle, Stalls when Put in Gear

A Chrysler type ECU conversion would have required changing the distributor.
That factory stuff from that era is very tight and there's not much latitude.
Map out the timing and compare with factory spec.
By that I mean measure timing vs rpm from the lowest rpm you can. Do every 200 or 250 rpm, or every degree or two, whichever is easier for you. Take it at least to 1800 rpm, and preferably as high as you feel comfortable.
Plot that on graph. That's the timing
Now take the initial and remove it. This will leave you the advance curve in crank rpm and degrees.
Compare with the FSM advance specs which is in distributor rpm and degrees.