welcome to the garage Joe, THE place to sort it all out
if the 63 frame had a step bed originally, the mounting holes on the frame for a fleet bed could well be different - the step beds are the same 55-66, and fleet beds fit the same frames, making the 60-66 fleets interchangeable ... don't quote me, but I believe the fleets all have at least 6 frame mount bolts, the longer ones 8, and there should be 'pads' on the bottom of the cross sills where they sit on the frame, the spacing [width] on those shouldn't be any different 60-66 ... sure your looking at the right holes?
pictures would definitely help some of the 60-66 experts see what yer up against
Bill
