Engine miss under load - ignition concerns

Hello all, hope you can help (I know you can).
I've read up on a couple dozen posts through the search function with people having various ignition troubles and I've learned allot thanks to all that have contributed.

My car and a bit of what I have checked so far:
1973 Duster
Stroked 360 Magnum based engine
MP electronic distributor and orange box with new harness between box and distributor.
Fresh rebuild but was miss firing before hand.
New Champion RC12YC plugs.
Firecore wires. (unknown age and I haven't measured resistance)
I do have a copy of the wire harness schematics and I believe everything is back as it should be. Was hacked up quite a bit and I removed all unused wires from other projects PO must have skillfully (sarcasm here) installed. I did leave the original ignition system wires in place though they are all dead ended - it used to be connected to a MP connection MSD like box at one time from the old photos I have of the car.

New ballast resistor
Battery holds good charge and car starts easily and idles OK after it warms up a bit (No choke on dual 4BBL setup with cam duration 236o @ 0.050)
Alternator main wire puts out 15V
Alternator field wire puts out 13.75V.
About 13.65V on feed side of ballast (all voltages with car idling @ 950RPM)
6.75V on ballast out to + side of coil and at + side of coil.
3.62V on negative side of coil. Was over 4.2 but tried grounding ECU better and it helped lower - side of coil voltage.

What I haven't checked:
Timing (I have to borrow a light but I have moved it about a bit. Pulled back from hard start a bit and it starts well, tried retarding more but ran worse, advanced timing to a hard start position and no change to miss under load)
Plug wire resistance
Reluctor clearance.

From what I have read my negative side of coil voltage may be too high which could be a bad ECU? (Orange box).

Have I given enough information to lead to a diagnosis or do I need to do the remaining tests and adjustments before pointing to the ECU?

I do have a complete MSD HVC 6601 ECU, Coil and Soft Touch system I could install but I'd rather save for another project but if the Duster needs it it needs it, LOL! Currently mocked up on a 340 engine......