The 70-72 frame rails are straight back in the rear area on the 4wd and kinda bow out around the coil springs on the 2wd. The 4wd will have leaf springs in the rear but the 2wd has trailing arms and coil springs. Most conversions I have seen keep the rear leafs, and put the front crossmember out of a 2wd truck in the front. I'm going from memory but I think that's right.
Good luck, Rg
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