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The 1965/66 F100 2WD frame is only different inre to Twin-I-Beam suspension and how the engines/trans' are mounted, behind the cab it's the same 1961/66.
9" rear axle and rear leaf springs same 1957/67 F100.
The only difference might have to do with the wheelbase. 1953/60 F100 with 6 1/2' bed: 110 inches. 1961/66 with 6 1/2' bed: 114 inches.
Frame width is the same. Back of cab to center of axle is same. Only problem might be the cross member mounts location. I'll bet some 1/4" flatbar could fix that real easy.