I'd call Sportsman's Supply and Gun World ( Harrisville) and see who does their work. I used to go to Joe McCrea before he died and he was excellent. I'm not sure if any of his guys are doing work on their own or not. I think I remember that one of Van's sons (Van's Sporting Goods) is a gunsmith. I think he's only open occasionally.
Another option is the B-Square saddle mount like the one at http://www.midwayusa.com/product/367653/...-1-2-and-3-blue . They're aluminum and you can easily install yourself by just popping out the trigger housing pins and putting the saddle on with the pins provided for it. Easy to take off too. I wouldn't buy from Midway but Cabelas, Gander Mt., and many sporting goods stores carry, especially for the 870, and they usually run $42-$45. I have one on a Winchester 1300 that I bought used several years ago with a red dot sight. I'm not a scope guy on a shotgun but it is my back-up turkey gun and it works perfectly.
Right now I have a B-square on the 870. I have another 870 that's drilled/tapped. The drilled/tapped one is just slimmer and better handling. I like that look better too, but thanks for the suggestion. Pete
Regarding the cantilever barrel, I did also think about that but I already have all the barrels I need and this is really going to be a turkey killer. If I was setting it up as a slugger, the cantilever would probably be more accurate. Thanks for the suggestion too. Pete