When I had my old '63 C10 longbed, I bent the frame when I backed into a telephone pole at about 35mph. I hydroplaned on a wet road and spun out.
I replaced it with a '65 frame and all was good. One thing you need to look at is the way the track bar under the bed connects. '65 is different from '63. It's not hard to make it work, but it is something to consider. When I moved the transmission crossmember rearward to switch from the Powerglide to the TH400, I reattached it with bolts.
I drove that truck for another 6 years before I sold it and I know the new owner drove it for at least another 3 years before he sold it. There were never any issues with the frame or crossmember. I used grade 5 bolts when I did it. Stronger than standard grade 3, but not as brittle as grade 8 when the frame flexes.
I'm glad you caught the rust issue before you finished the frame and installed the body. It's a lot easier to do the switch at this point.
