Since the drum center is relatively thin, and not hardened, just drill it to fit the new 5 lug pattern. Even if there's a slight overlap with one of the holes, it shouldn't compromise the braking effect. I'd suggest making a template out of thin sheet metal that will fit inside the center of the drum, and rotate it around to find the area that will have the best stud engagement, then drill there. Cutting the final hole diameter with a 4-flute end mill will give a much more accurate hole than trying to use a drill bit. Drill the holes a little undersize with a bit, then switch to an end mill for the final cuts.
Jerry
