First time I've ran into this, I bought a run and drive dune buggy project that has a 1956 swing axle chassis. After I stripped every part off the chassis I found a couple of 1/8" bubbles on the underside of a frame horn. I used a body hammer to " sound" that area and soon had Swiss cheese on both of the lower side of the frame horns. To inspect the damage I cut a 1 x 12" opening finding 1" deep piles of rust............. The upper frame horn sections and the rest of the chassis are solid. Point being I would suggest that anyone looking at a Speedster donor chassis to carefully inspect the lower side of the frame horns (hammer in hand). I'll repair / renew by cutting out the rest of the lower frame horns then tacking 11gauge steel applying torch heat rolling it over as I tack it in place then go back and run a solid bead.
