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Go to the saiga forum and find the saiga builder or gunsmith closest to you. What you want done is really simple, no need to send it off to one of the best custom builders and wait months and pay hundreds more for something a local smith could have ready in a few hours for far less.
Elk River Tool & Die. Hector is a great guy to work with and charges a more than fair price.
Tac 47 in Houston does excellent work and Keith is a great guy to deal with. They are not the very lowest priced conversion company, but they stand behind their work.
If you ever decide to go really high end, Lone Star Arms is amazing. Mike is a jeweler and his work on the Saiga 12 bolt is awesome. It is mirror like and you could eat off of it. I have bought two guns from him and he gets my highest recommendation (with Tac 47 right there, too).
I have bought about a dozen S-12s over the last few years and these two companies are my favorite. I have owned S-12s converted by almost all of the big names in the industry, and if I were to have another Saiga S-12 converted I would go with either Keith or Mike.
Oh, I forgot that Jack Travers, of JT Engineering, does a nice, rather basic, conversion for a pretty reasonable price. Check him out on the Saiga 12 forum. I bought one Saiga S-12 from him and I was very impressed with it.
I had mine done my Michael Rogers of Lone Star Arms. I am very satisfied with the craftsmanship and value of his work.