Rookie mechanic: trouble shooting advice requested

Hey, guys.
I have always done basic maintenance on my cars, occasionally getting a bit deeper but generally the tasks that I've done were without guesswork. For example, I'm happy to replace brakes, drop the transmission to swap out torque converter, etc. If I know how things are supposed to go, I'll go for it.

I did much of my Magnum swap on my own but I had help from some friends with some of the (few) trickier parts.

When it comes to diagnosing...I'm weak. :D

So my (carbureted) "Magnum 300" 5.9L suddenly won't start. Last week, I fired it up and started immediately (as usual)... stumbled and stalled. Not the first time this has happened. I fired it up again, it ran for a moment, coughed and stalled again. So...I assumed it was flooded. I did the usual "hold the pedal to the floor"/flooded engine start-up procedure that has worked in the past...no luck. No ignition whatsoever.

I pulled out each plug, saw that they were wet with fuel, dried each of them off...tried again. Nothing.

Is there fuel? Yep. In addition to the wet spark plugs, the Demon carb has windows on the float bowls...I can see that they're half full, as usual.

Is there spark? Well, I pulled one of the plugs, hooked it back up to the wire, set it aside and turned the key- nothing- I SHOULD see a spark, right?

The MSD coil is pretty new (~500 miles). I checked the connections to the coil, swapped in a different coil to distributor wire- nothing.

The contacts in the cap looked ok but a bit funky so just in case, I grabbed another cap and tried that one- no difference. The rotor looks ok...didn't try a fresh one yet but something tells me that's not the problem.

The ignition box on the firewall and the ballast resistor are also new (~500 miles), the car has a Mopar Performance "chrome" ignition controller. The connection to the box seems to be solid. I don't have another one to check but I've never heard of one of these spontaneously failing.

What should I check next? The wiring harness is original and pretty crusty but I'm not quite prepared for the cost of a new harness at the moment...so I'd like to avoid a complete harness swap.

Any advice would be extremely welcome.

-Mike