I think the frames from 61-66 were all the same in relation to long beds, short beds, fleet sides, step sides. The 4x4 frame may have had an extra hole or two but should not be any different structurally. Take a picture of yours and find an old beater in a salvage yard to compare it against. There is a vintage truck salvage yard down in Pasadena, Tx you might want to locate. I do not recall the name at this time.
You might want to obtain a factory assembly manual for your truck. It will have all the detailed dimensions for the frame. Maybe a local frame shop and fix your original.
