Personally I would not drill and tap the frame. Although it might work, you are risking weakening the dropouts. P-clamps or the QR adapter you mentioned are plenty strong for carrying a small load. If you are planning on carrying a lot of weight on the back you need to use a different bike for that. The goal of this bike was light weight tubing so using it for a large amount of groceries or touring is not a good idea. It is just not the right tool for the job. Which might give you an excuse to find a different bike with rack eyelets

By the way, yes, those are early 90s decals. Very similar to a '93 Tachyon I once had.