I've got gas in my oil!

I have a 1980 Ford F-250, with a 400-M motor, and i noticed that i have an extremely large amount of gas in my oil, and i have no idea where it's coming from. I thought it was the carb's power valve, but that wasn't it, and i also changed the fuel pump, and that didn't fix it either. The truck set for about a year and a half with out being turned over, and it never had gas in the oil before. But now it seems to just appear in the oil by the quart? What could it be, any help would be greatly appreciated.