A couple of thoughts:
The SMLE magazine will work well without mods for tapered cartridges up to, at least 35 Winchester.
The 35 Win, BTW, is a great caliber and much more capable than most. Larger straight cased calibers require a single stack mod to feed nicely...not trivial. Also, the magazine is a bit short , but still works for 35 and 405 Win.

For open sight SMLEs, the Parker Hale PH 16 receiver sight can be adapted, is much nicer than the Williams, and seems to be regularly listed on the auction sites. It's Brit origins and all steel construction are aesthetic necessities if you're going to the trouble and expense of making a nice sporting rifle out of a SMLE.

Otto