Stainless steel is great, except when it comes to weight!
The 640 tips the scale at just under 23 ounces which is about what all "snub" all-steel 357s end up weighing.
I was under the impression you wanted to retain the ballistic advantage of the 357 magnum, and IF you're welded to stainless steel you might as well stay with 357, since all-steel 38s weigh pretty much the same.
However, it seems to me a BETTER answer for you is one of the modern, hi-tek coatings on the gun you already have and prefer!
While you CAN hand it over to someone to do a super job, you can also DIY with surprising results. Several years back, while experimenting with all sorts of coatings and even paint, I sprayed a bare aluminum AR-15 lower with PLASTIDIP of all things! I figured, why not, if I don't like it I'll just peel it off! Thing is, that finish is still there! Five years down and that plastidip, spray-on finish is still stuck to that formerly bare-aluminum receiver - and it's been in service the whole time! Since Plastidip is a sort of "rubber" the surface is a satin matte - black in this case, and completely impervious to salts such as those in sweat.
Even more years back I sprayed some OD Thermoset resin epoxy I found at Brownells onto a bare stainless 1911 frame I had just machined. That stuff went on and hardened into a finish so tough it's still there! I put the same stuff on a Sig P228 frame build - OD green - beautiful color, and to this day it's still there - even on the rails!
Duracoat offers all sorts of "kits" complete with how-to videos. A little disassembly, a little masking, spray, bake it, and it's gtg!
p.s. I even sprayed the OD green Thermoset resin on all the metal parts of my Mini-14! Baked it in the oven...that was 20 years ago and the finish today is harder and more difficult to even make a mark on than new! Even where ejecting brass bounced off the flat part of the operating slide, there are brass-colored marks, but the OD green surface is solid!