I agree with CP.. Sounds more like a leaky gasket. What does the intake's #7 port look like? If it's clean, the oil has to start somewhere, and it's pulling it past the gasket. Iron guides? Not sure why i'd bother using them if you were drilling out and replacing. Bt, if they are in there, more than likely they are sealing. If they were that loose, I think you'd have some other bad issues, and it would be more than one hole. Careful disassembly should tell you. Clean of the gaskets and end rails, lay the intake on the longblock with a trouble light behind it, and look at the fit. You should see no daylight on the intake port mating flange areas, and about 1/8" or less on the end rails.