Agreed. High mileage engines can be a crap-shoot when repairing them. You only get one "wear pattern" for the rings. The renewed heads will increase pressure, potentially altering that pattern. It will begin to smoke rather soon...
Get another mechanic. There is no relationship with seals and fuel type. STRICTLY "myth and superstition". FWIW, many "modern" techs, though fully qualified for repairing modern cars, are often in "over their head" with older cars. While one assumes the older stuff is less complex (basically true), they have a completely different "set" of strengths and weaknesses than modern engines/cars have.
If the ONLY thing "wrong" is the head gaskets, milling the heads for "flat" shouldn't create the pressure "issue" with the rings, UNLESS a significant amount has to be removed (in excess of .010").
Best of luck. If any further questions, be sure to ask!
