Dielectric grease is your friend. Put a thin layer around the inside of the boots at the distributor cap, and at the coil too.

Are you running points & condenser? A points distributor w/ say, Pertronix Ignitor? High output ignition coil?

Couple things I ran into with my slick fwiw. One is the plug wires have to be high quality. I didn't cheap out on them, but I had less than stellar results with "roll yer own" plug wires. Maybe I didn't crimp them well enough. Using a high output ignition coil like the Flamethrower with direct full time battery voltage puts the old school points distributors under the test. Notice how much bigger in diameter the distributor caps are when HEI/Duraspark came along. This summer we had several weeks of very high humidity, and the garage was like a sauna. The distributor #6 boot wire was arcing to ground through the distributor cap straps, that secure the cap. The terminals at the coil both + and - were arcing as well. It wasn't stumbling, but it was a light show in all that humidity. I had a similar problem with old plug wires.

A new distributor cap actually made things worse, a better spark, sure, but still arcing outside. Dielectric grease helps but really shouldn't be necessary.

OEM Motorcraft wires really helped eliminate that. I bought an old school ignition analyzer that displays both primary and secondary waveforms, superimposed & parade etc, to check it out and really dial everything in. Another thing "they" did, the earlier black pre-smog distributor rotors have a longer tip than the later blue ones. The rotor - terminal gap inside the cap is almost a 1/4" in later years. This increases the coil firing voltage quite a bit but it isn't really what the earlier engines want. Everything in the ignition works together and is cumulative. Resistor plugs versus non-resistor plugs. Resistor wires versus non-resistor wires. Resistor coil wire versus non-resistor coil wire. Rotor gap. Plug gap. Different combinations really affect how it runs, and how susceptible it will be to crossfire & arcing both inside and outside the cap, etc. Today I took a pretty long drive in a heavy mist light rain, just enough where windshield wipers needed to be on constantly. It ran great, but it was a real pain to get it there! Ended up using non-resistor NGK plugs gapped to factory spec of 030", genuine Motorcraft resistor wires, and a close rotor gap w/ Pertronix Ignitor and Flamethrower w/ no ballast.

If you can find someone with an ignition analyzer to borrow it will show what's what right away, and whether coil firing voltage is spiking, defective plugs, excessive plug gap, or defective wires, etc.

Try a spray bottle filled with water and a darkened garage at night and spray down wires and cap &c at idle and see if the stumbling or misfire can be reproduced. Look for arcing to ground. A lot of people jack up the plug gap too much, this increases the coil firing voltage, and increases wear of ignition components, especially rotor. I noticed on my slick a half moon carbon track etching or arcing through the rotor itself, voltage was blasting through the rotor to the distributor shaft. High voltage is wacky stuff, and, if it can find an easier path to ground than the plug gap it will take it.