Dane: you may be able to find a tutorial on this in one of my Hawken threads. If you cannot, I can go through my files and find the info for you.
In short, you file a flat on the rib down to the concave. Then you file a flat on the pipe right down until you get a gap where you break through. When you put the pipe in the rib's filed flat, the concave groove of the rib will be continuous with the hole through the pipe, and there will be very little gap along the side joints...the pipe will look like it grew out of the sides of the rib.
I use an aluminum rod through the pipes to hold them in the concave groove of the rib without clamps...tin the pipe's flat and the rib's flat, and heat them as a unit until the solder flows. I use electrical solder (contains 2% silver) as it flows so well, is super strong, and if easy to clean up. This solder is only about 1/16" diameter wire and responds nicely to ordinary paste flux.
This system has worked on I don't know how many rifles, and I then use the pipes as a guide for the rod drill to create the hole in the forestock...it's practically foolproof.