help me find an electrical gremlin, please

Ditto on HEI is simple, cheap, and better. I put the GM 8-pin module & coil on my 64 slant, ~$20 in junkyard parts. Same parts as shown in 73Abodee's link. You could start wiring that up in parallel and switch over when convenient.

Since you already wired-in the Mopar ignition, might be easier to try to fix that. First run a good, dedicated ground to the orange box. Try swapping boxes and coils. Then test the distributor pickup. Of course you need spare parts for this. Testing spark is easy if you spin the distributor shaft by hand. You could pull yours since a SB is easy to put back the same. Mark where the rotor is pointing and don't turn over the engine (pull yellow wire off starter relay). For checking the spark, try an in-line spark tester ($4 at Harbor Freight) to view while the engine is running. Similar is to use a clamp-on timing light and insure it flashes evenly when clipped to all cylinder wires and to the coil wire (8x faster).

If all this is too tedious, buy a $45 ready-to-run distributor with built-in HEI module and a $20 E-core coil. All you need then is +12V from your ignition switch. Of course, even that won't work if the later is your problem (bad switch, wiring, or bulkhead connector).