Problem - gas where gas shouldn't be

[Cornmeal]

Hi all! This is my first post to the message board, and I have a problem with gasoline getting to places it shouldn't be.

Background: Bought a 67 GMC 'fixer upper'. Previous owner said he had just rebuilt the engine, but it ran kind of rough and looked kind of greasy, so I didn't really believe him - I planned to replace the engine anyway. Ran it for a few short trips, and it smelt pretty rich. Drained the oil pan, and there was a lot of gasoline mixed in with the oil. I figured it was a ruptured fuel pump, but didn't think much of it. As planned, I replaced the engine with a new Goodwrench 350... new fuel pump... new Edelbrock Performer 1405. Ran great for a few short trips. Then not so great. I checked the spark plugs, and they were fouled. Put new spark plugs in, leaned the mixture slightly, and seemed to run great again on a short (5 mile) trip. Then I parked the truck in driveway for a couple of weeks (slight incline, nose down).

The problem:
Went to start the truck this morning, and it wouldn't turn. Couldn't figure out why until I took a couple of spark plugs out and a cup or two of gas came out of the spark plug holes! I drained the oil pan and it was filled with gas too?!

My first thought was that maybe the carb had a stuck float, or that maybe the mech fuel pump was at too high of a pressure, but the carb is new and I find it odd that it showed similar symptoms with a different carb, pump, and engine.
I did notice that the gas cap is not original, and wondered if maybe it was not releasing pressure as the temperatures go up and down. Could that cause enough pressure to force gas past the carb? Any other ideas?

Thanks for any input.