I'm sure you're aware that butt transplants typically utilize an oversize blank that is grafted onto a sound head and grip section. But I've always been intrigued with the idea of attempting what you are proposing here. A number of good ideas have been forwarded so far. I like Bill's idea of using a sled on a table saw to make your cuts. A couple well matched and fitted mortises with a hardwood plywood tenon for maximum strength in the joint rather than dowels or all-thread. I think Titebond wood glue is fantastic stuff, but I'd go with the West System epoxy suggested by Mergus, simply because it will be tough to get a perfectly mated joint in the mortise and tenon. Using a milling machine would be best for machining the mortises. Epoxy is a much better gap filler than Titebond wood glue, and being internal, no one will ever see it. I like craigd's idea of sneaking up on the final mating surfaces by cutting a bit oversize, and taking it down to final perfect fit with a belt or disc sander. The joint should ideally end up hidden in the bottom of a checkering groove on both sides so that very little glue line is visible. A darker alkanet type stain hides a glue joint better than a lighter finish.
An interesting and challenging project Gil. I hope it turns out well, and I hope we get to see the results.
