Regarding welding the toggle link for your original Winchester 1873, I believe you can get the job done. DO NOT get rid of the broken parts!
However, my advice is that you find a good gunsmith who is familiar with that gun who is also a TIG welder. If you find that combination (gunsmith, win73 man, tig welder), he can repair the part by welding it and then dressing it down to spec. It probably would be smart to find an original part to replace it anyway and keep the one you bought for a spare. If the gunsmith is so equippped, have him Magnaflux the "new" part you purchased to check for cracks. I say use the repaired part in your gun to test it to see if it will hold up under SASS use. If it breaks, you want it to break right away, not three years from now when you need it.