It cant be too clean. You must remove all old gasket. I use razor blades and a dye grinder with a plastic bristle disc. I cover valley with clean rags then pour atf over the valley to rinse the loose debris into the pan. Always change oil and filter before starting. Pay close attention to the had places to get to like around the alignment dowels.

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1964 Galaxie
1967 Cougar
1968 F250 Camper Special