I hear in the late 80's to around 96 Ford used Identical frames on their F series between 2wd and 4x4 to save money and the frame had even t case/axle bolt holes, but I'm not sure about a 65, I also know that before the sixties ford 4x4 were all outsourced and custom builds from 2wd's. Its just all built on heresay thats the problem.

What year what model F series dually do you have?