I just read this post and wondered how you are coming? Just my thoughts for what they are worth but if it was my rifle I would want an integral front base, rear island with integral flat leaf to file in and and integral sling swivel large enough to take the old hooks. An integral recoil lug would not be required, a silver soldered dovetailed one is fine, if the rifle is inletted and bedded properly.

I have a .585 Inyati by Ralf Martini that has two integral recoil lugs, an island rear a front base and an integral sling swivel. The barrel blank started 4 inches in diameter.

There are pictures somewhere on this site.