I had success using glue to re-attach the rear sight. "GOOP" holds forever, but you can still remove the sight without damage if you really want to. Do NOT use epoxy, JB Weld, etc. They are not forgiving if you make a mistake
Williams peeps are very hard to find right now ! There are several ways to put the original wedged sight back on. Sometimes, by using a helper to hold the wedges in place, you can tap it on like you are trying. I lay the sight on the edge of a table & file just a little from each side by hand, where it doesn't fit as tightly. You can also file a slight angle on each side that helps the sight slide onto the grooves in the wedges. I have watched people lay the wedges in place, put the sight right over top of the wedges, & whack it down with a mallet/block of wood with a hammer. That looks scary, but it works. I have never seen barrel separation using this method. In my opinion, the sight " causing " barrel separation is exaggerated. Sights can get hit on these rifles from rough treatment, tossing the rifle in the truck, dropping it, & some scope mounts can cause separation.
How long does it take to do that? I bet quite awhile?
The reason I asked is because I recently did some filing on a Williams 5D-SH (anodized aluminum) and that took forever. Brand new Bas#ard mill file, too. I was using the method Steve described. I figure the steel on those sights must be even harder than the anodized aluminum.