I've put together a little example page to show where the seatbelt mount points are in later Midgets.
http://millenniumtree.com/share/midget/index.html
Hope that helps. We bought our '69 Midget without seatbelts. The first and best $100 we spent was to buy 3-point belts. (# 222-206)
The mount points hadn't been used in some time and the lower ones were quite rusty. I cleaned the threads out with a wire brush as best I could, squirted some liquid wrench in there, then slowly tightened/loosened the seatbelt bolts repeatedly until they threaded all the way in. (poor man's thread tap) If you go slowly and carefully, you shouldn't hurt either the bolt or the mount.
The seatbelt kit came with all the nuts/bolts we needed.