G'day: I had to replace the frame on my '76
Boler 1300 a couple of years ago, due to total failure of the original. While at it, I also replaced the clapped-out
axle. One of the design weaknesses of the old
Boler was the use of the screws to retain the body to the frame. I up-sized the frame to 0.125 wall thickness, and added mounting tab similar to the original 4 on both sides of the frame wherever possible--14 in all. Before removing the original frame I marked the new tab locations wherever there was the imbedded plank floor to provide load support. For mounting bolts I used 5/16 stainless steel with nyloc nuts, and used large diameter fender washers internally. The old frame was then used as a template for the new.
Incidentally, during the rebuild, I also determined that part of the problem with the door fit was due to the
weight of the body shell being carried cantilevered outside the frame--the misfit was not all door problem. The sag could not be corrected in the body, but did improve slightly with the added tabs.
With your access to the welding, I would suggest you forget about reinforcing an antiquated insufficient original frame with a new heavier duty unit. No point attempting to reinforce the old frame- the reinforcement will only transfer the flexing to another weak area. I speak from experience, that's what happened to me.