This looks like an additional box tube was welded in on the lower tube to closer to the front. On my 1986 the bottom tube ended there. I welded an additional tube myself here.
A 1/8" plate scarfed over this spot would be nice. I would clean off the rust first since the rust will make it hard to weld. Then weld the "crack and grind it down. Prime the metal to slow down runs between the tube and the scarf plate. Sand off the primer where the welds will be made and weld it up. 1/8" plate is thicker than the original rectangular tubes and the same as the newer tubes.
I would taper the scarf plate to end in the middle of the tube since there is no bending stress there. A plate over the top and bottom would not be a bad idea either.
I tapered this rectangular tube to have less stress concentration at that point.
If you look closely at the extreme right you can see where the new tube is welded into the original bottom tube and brought forward.
(There is a little misalignment at the front, but eh coupler came out in exactly the correct point)
You can see where this tube is tapered under the new part welded in at the old cross beam. This new tube is structural steel 10 gauge (1/8" think).
Here is a similar thing done on the passenger side. I added two cross beams since I was adding the front bath ahead of it.
I brought the frame forward to the end of the shell to make room for the bath and from there the 50* angle to the new Bulldog coupler. There are more additions to come up here (like the mini-split compressor and propane
Note the splayed patch across the bend and the taper to the center of the tube. The extra cross member is welded at the bend to stop the squeezing stress and reinforce the weld.