WK,
You going the full-bore specialty turkey gun route, or shooting slugs? If it's slugs, ask him about drilling and tapping a short screw thru the top of the receiver into the barrel extension while he's at it, to lock down the barrel too. Won't matter so much with shot loads.
Ask the 'smith if he'd rather pick out the base, or maybe make one from stock. Hole spacing may be a consideration given the contour of the top of the receiver on the inside. The cantilevered mount barrel might be an idea if you can find one that is smoothbore for turkeys. A lot of those are rifled, for shooting sabot slugs. Dunno a lot about those myself, never messed with them much. There's an old two-piece cantilever barrel that came on my 11-87 that is smoothbore, I took the cantilever arm off and mounted a 1.5X Burris EER 'scope scout style on the base and that's my only experience with them.
Stay safe,
lpl/nc