I found some Belt Drive dropouts that are available:
https://www.paragonmachineworks.com/s...-261333-3.aspx

This might be a dumb question, but can you weld stainless steel dropouts to a chromoly frame? What parameters would be necessary for these dropouts to fit besides the necessary width of the chainstay and seatstay tubes? Must the seat stay be of a certain angle to the chainstay? Thanks in advance

Yeah, you can either tig weld it or silver braze it. I just finished a belt-driven mountain bike and used the same style of dropouts you are looking at, not sure if you are using the slotted or breezer-style, I used the slotted. It was the first time I had played with this style of build so I took a lot of time to make sure I got it right and it turned out ok. You can view it on my blog. https://meechcustombikes.blogspot.com There is a photo of the finished bike and also a couple of close-ups during the build. The customer said the belt-drive is super-smooth, I haven't gotten to ride it yet.

Nice Job. Finally a simple system.

"Must the seat stay be of a certain angle to the chainstay? "

No because the bolt is a pivot point, or that is how it looks to me.