I'm attempting to turn my '86 F-250 4x4 into a dually flatbed as we speak. I'm using an '89 F350 dually 2 wheel drive as a donor. The dually frame is in fact narrower. So are the rear springs, 2 1/2" versus 3", and the distance between the spring perches is a lot narrower. The actual axle tubes themselves are longer by about 2" on the dually. When I tried to put the dually wheel on the F250, it rubs on the spring. So now I'm trying to figure out what to do. I'm considering heavy duty bolt on wheel spacers, the kind that bolt to the hub, then your wheel bolts to the spacer, not the cheapy sandwich type spacers. Swapping the axle is almost out of the question since I'd have to weld on new perches and change the gears.