Something made me scratch my head

You'd never feel ONLY the difference in compression the plugs could have cause "by the seat of your pants.

Here's one possibility:

Do I understand --that you changed the plugs, the coil, and the box?

Here's one possibility---the "insides" of a Mopar box work JUST like points, that is--

the box switches the coil on/ off AND IT HAS a "condenser" (capacitor) inside the box JUST like your old points distributor did.

'S also possible - assuming you changed the coil - that it was a little on the lame side, too.

So what else did you change? Plug wires?

It's very possible that the old box, overheating and puking guts--also deteriorated the capacitor, causing reduced spark