I´ve waterproofed a few distributors. The best methode, seemed to be a fine bead of silicon sealer at every seam. The technique that seemed to work the best, was when the silicon was sandwhiched, between the rubber piece and part to be sealed. A dab on the drain hole in the bottom of the distributor, a small bead around where the cap seals and where the cables enter. A small bead around the plug cable distributor towers and on the plugs themselves, about half way down. Where the wire goes into the boot, I also sealed with a small bead of silicon. Same with the coil, high voltage cable.
The down side is when you have a problem, and want to troubleshoot. Cleanup before reaplying, the silicon can be a pain.
I´ve completely submerged, distributors, sealed with this methode and had no loss of spark.
Next best methode is just to seal the distributor with silcon sealer and spary all the boots, with a silicon spray, for rubber (rubber softener/conditioner). Soft cables and boots seal better.
Never had much trouble with, a non vented distributor cap. Vented caps, always seemed to corrode faster than none vented caps anyway. On the 4.0 motor the Ford non- vented cap, seems to last longer than the OEM type jeep vented cap (and was a buck or two cheaper)..