The back 2 feet of my frame was rotten, including the areas where the shackles mount (not good). I sawed some rear frame bits from an Isuzu truck at work and started welding: New bushings are in-the-mail, along with rear brake parts (for those that spotted the brake fluid leaking from the left drum)
That pic was with the car on the ground and 2 FE heads and a cast-iron intake in the trunk. There is also no bushing in the frame-eye yet.
I went around the yard at work measuring frames and the Isuzu was closest... only 1/4" taller. It made for a nice underlay/Z action under the rail. I will still plate the inner side of the frame after dressing the inner welds down a bit. I am all about overkill when it comes to frame.
Good game plan and thumbs up on figuring out a solution for your problem. Nice job on the welding too.