electrical or carb issue mmm

I don't know. Since you are running a points ignition I would be VERY tempted to yank the MSD out of there at least temporarily. High on my list of suspects is the coil itself. Have you checked starting spark to get a feel for this?

(On that note, it is IMPORTANT, if you still have a ballast resistor in circuit, to check cranking spark BY USING THE KEY to crank the engine, and NOT jumpering the start relay. This is because you won't have the resistor bypass circuit in affect.)

I don't believe you WANT the radio suppression cap (condenser) on the positive side of the coil with an MSD, so if you have one, I'd remove it.

(So far as the condenser in the distributor, I don't believe it's needed for MSD, but probably won't hurt. It certainly is needed for a stock points/ coil)

Check your original "dark blue" "ignition run" wire supplying the ignition system. This is the wire coming from the key/ switch, out of the bulkhead, which originally supplied the "key" side of the ballast, and also the ignition terminal on the ballast.

Check this key in "run" engine OFF. Stick one meter probe on that wire, the other probe on the battery positive. This will show voltage drop through that circuit, and you should read LESS than .5 (1/2) volt. The less the better, and even 1/2 is generous

That much or more indicates a drop somewhere, most likely bulkhead connector, the ignition switch connector or the switch itself.

Any thoughts on condition of fuel, tank, fuel line? Have you had the carb apart to inspect for dirt/ rust? How does the filter look in this regard?