i SAS'd an '86 2wd and ended up using some jigs i got from marlin and cutting with the torch. the holes were a bit wonky, but i tacked and measured to make sure they were straight. right after i welded them i seen someone holesaw 4 frame plates, weld the plates to the frame (over the torch cut holes), and then weld the tubes to the plate. it looked much better, and was probably stronger than the gap filling a ultimately had to do.
that sounds like a good idea. thanks.