HokieJeep, it might be a stupid question but, do the screw holes on the cap adapter line up with the old distributor cap? If they do, maybe rotating the cap adapter 180 degrees will work. I know when I did mine I tried it one way and it didn't fit. Rotating it corrected the problem. By the way, what year Ford is the cap adapter for?
on my 1980 I used the following part numbers. They are from "Car Quest" auto parts but the parts are Standard Ignition, a good name. The numbers should be able to be cross referenced by the auto parts guys.
The main problem I was seeing was that when the cap adaptor was screwed on, there were gaps between the base of the distributor and the cap adaptor. The bottom of the cap adaptor was not molded to fit perfectly around the perimeter of the distributor like the original cap does. I would imagine this would cause havoc when water splashed up in there (or dew in the morning for that matter)