Hi All,
I'm in the process of replacing a 24" or so section of rear frame. This is from the rear bumper bracket/tail panel forward. I have cut out the old section and am preparing to cut the replacement section from a rust free donor rail. I am thinking what I will do is fab up 3 pieces of 1/4" steel that I would weld into the inside 3 walls of the car frame, there would be a couple of three inches sticking out of the frame. I would weld these through 3/8" holes drilled through the car frame. Then. Drill like holes in the replacement section . Fit the replacement section over the 1/4" steel and butting the two frames together, align all properly with a jig, weld through the 3/8" holes of the replacement section , then butt weld the replacement section to the car frame stub.
My thinking is the 1/4" plates will help with alignment and allow a better weld penetration. Hope I have explained this in an understandable way. Am I missing anything? Or creating a problem going forward? The trunk floor replacement after this will be another post.
Thanks for looking.

better off replacing whole frame rail. why cut up 2 rails ? unless the donor has bad spots.

Ok thanks

if you do the repair as you described, 12ga, or 1/8" steel would be plenty thick for those plates.

That makes sense . Thanks

I have to agree that if the donor rail is rust free the entire length replace the whole thing. That way you are basically redoing the welds at the factory locations. If you are confident your rails have not shifted you can make braces up off the whole floor based on the original rails to locate the new ones. As far as I know the only critical location for the rear frame rails is the rear spring rear mount perch. Get that located and it should be good.
As it sounds like you are replacing the trunk floor you will be drilling out and redoing most of the welds you need to do to replace the whole thing.
The other thing that concerns me about the internal doubler is bare metal inside the overlap area being an excellent place for rust to start. Plus you are dumping a lot of heat into that location which can result in distortion.
I have been going through this mental exercise on our 70 RR. Rear frame rails are one of the first pieces, hence my interest in clicking on your post. Ours are trashed, one is cracked over 80% of its girth. In general I am planning on replacing entire panels as much as possible. Much less welding. Imagine trying to seam weld an entire quarter panel skin and not warp it....
Finally starting on parts ordering etc. When done we should have about a 2015 1970 RR if you average the old versus new metal.
Good luck with whatever approach you choose.

Doing the splice is fine. You can use a section of the replacement rail as a sleeve insert. Cut it in half to narrow it, plug weld it in like you planned. Standard collision repair on a unibody.

Great proposal there. Makes sense, since that section to replace is usually easiest (access). When you get up towards the spring hanger area, it's more involved & critical. Rust? When done welded/painted or undercoated: don't worry unless you're uses car everyday rain/snow/mud/slush. More likely it may get 'wet' when you wash the car.

Thanks for all of the helpful responses.