McMaster-Carr is a decent retailer, MSC Industrial Supply is better.
Probably neither will have a half-thickness nut in a 7/16 x 20, although I do not know that. as I do not need that size.
Stuff like that generally has to be custom-made. We used to put body screws in our carburetor kits, and odd sizes (13/16 instead of 3/4 or 1 inch) generally had to be special-ordered with a minimum of from 5000 up. We quit putting body screws in our kits when the U.S. suppliers went out of business. I will not use the junk.
Not a big fan of stainless fasteners; but if you are, don't EVER use them with a zinc alloy a.k.a. pot metal part (i.e. carburetor body) unless you wish to buy a new carburetor every time yours needs rebuilding. Or you just REALLY enjoy heli-coiling every screw hole!
If you find the fasteners in the correct size, but wrong finish; you can chemically strip the finish and apply the black oxide, assuming you are doing a restoration where finish is important.
Jon.
