If you are running the stock fuel pump, it could be leaking into the
oil pan. I had that happen on my 1965 390 and the dealer told me something inside the fuelpump had split and was letting gas through. I didn't notice any difference in how it ran, just when I checked the
oil I had gas in it. Fortunately I check my
oil almost every time I drive so I had no damage from the gas.
Ron