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If you only need the rear portion of the frame fixed, I spliced the frame on my 78 150 4x4 with a rear section from a 2wd about 12 years ago - I spliced it 1/2 way under the cab with fishplates, welded and bolted, etc. You have to change the spring hangers as the 2wd leaf springs are not 3" like the 4x4. That was easy, just knock out rivets, drill out to 1/2" and replace with frame bolts/nuts.
If your front part needs replaced, I think the 2wd is different at least at the front cross member, where the steering box bolts on, etc. I'm not 100% positive that the rails themselves are different, but I'd bet they are with the different front crossmember (which is welded in, not riveted like the others.)