If the pan has been used before the mounting holes are probably dimpled and if so turn the pain upside down and find a narrow piece of wood and place it under each hole. Take a ball peen hammer and gently tap around each hole until the area is flush. After scuffing the finish I'd wipe it down with some sort of solvent, I like to use brake parts cleaner, but make sure you're in a well ventilated area and don't breathe the fumes. Do the same thing to the mating surface on the tranny to eliminate any residue. I like to gather all the mounting bolts and with a thin layer of Pematex #2 coat the mounting are of the pan, install the gasket and use the bolts to hold it in place. I also use plastic clothes pins to keep the gasket from slipping and after the Permatex has a chance to set up you're ready to install. Put all the bolts in finger tight and use some kind of sequence to tighten them to spec.