I can endorse those Fumato valves. I have been very happy with mine on a variety of applications.
My concern is that getting the oil pan threads clean enough for the epoxy to stick could be challenging. Considering the extreme stripping you describe, there are probably no remaining threads of any consequence on the [softer] aluminum pan. There is a lot of potential threaded surface area to stick to in this case, but I wonder if the adaptor has thread length even approaching the unusually long drain plug on these. I suppose you can hedge your bet by packing some additional epoxy around the outside joint of the adaptor if you can get the surface clean enough, and roughed up with some sandpaper.
1) You 'tip' the car in such a manner that the residual oil is not inclined to run out.
2) Clean the pan threads carefully with brake cleaner solvent and perhaps scrub inside there with a 3M pad wrapped around a dowel or screwdriver tip. Maybe a toothbrush or test-tube bottle brush? Perhaps even better, a stiff wire brush used for cleaning 1/2 inch copper pipe fittings? Clean is good.
3) Once clean, if you have a Dremel tool (or equiv.), you could cut some small horizontal grooves or drilled holes inside the threaded tube on the pan to create something extra for the epoxy to 'bite' on.
4) Flush out any residual debris from the cleaning/cutting process by running a quart of clean oil through before filling.
Retapping would certainly be better insurance, but I also tend to place a great deal of faith in the magic that is epoxy.
Best of luck.
BTW, this problem can be easily prevented using teflon tape.
1986 300E 5-Speed 240k mi.