I've used type 3 (4 bolt) rear drums for better braking on the backs of 2 cars; a Cal look bug (135/15's front with Ghia discs, 185/70-15's rear) and a Baja bug (again, a substantially larger tire) with great success. Yes, they did make the rear track (on both cars) about 5/8" wider on each side. I believe Thing rear drums and brakes are similar, but don't quote me on that. With the right offset I think it will work.
When you're mocking it up, note that the upper suspension stops can be trimmed, if necessary, and I've heard of people using button-head bolts for the springplates for clearance. If your car is a swingaxle, there are shorter and longer axles and tubes; the later ones ('68 and newer) are about 1 3/4" longer than '60 (or 61; sorry, can't remember which) to '66, and '67 is one year only and in the middle. 185/70's on custom 6" Sprint Stars (mounting surface moved out so the front spacing was the same as a 4 1/2" wheel), early axles and tubes with the type 3 rear drums on my '72 (Canadian edition, so it still had swing axles) bug and they sat in the fenders just like stock. If it's irs, the trailing arms can be narrowed up to about 1 1/4".
Hope this helps. Al