nice BB dart ...crappy rear frame rails..broken mount

Caps are welding good metal over old rotten metal. Not a good combo IMO. I've done a couple set of partial rail replacement's in B-Bodies (mid axle hump back), and would recommend you just cut the old garbage out and replace with new.

As far as the complexity of the repair, it's not terribly hard. First thing you need to do is clean those rails down to bare metal to see where your good metal actually starts. Along with others, i'd guess someone was tying to hide problems with undercoating. If in fact it's isolated to the very rear it wont be too hard of a repair because you'll just have to worry about getting the proper length, so the shackle mounts are in the right position, as well as the butting up to the rear crossmember. You'd have a decent section of the rear rail to guide/template the replacement metal with. Replacing a rail further up or a full rail is a different ballgame. For that you need to tally in rail orientation up & down, side to side and front to back. Get's tricky unless you have a jig or are stealthy with a plumb bob. Either way....sucks to hear. Never fun playing with frame rails, especially on a car that's pretty decent in all the other aspects of the chassis.