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The frames are not identical, the rear leaf spring length differs but that does not keep you from mounting a 56 cab on a slant cab frame. And as Jaye has pointed out, the early frame has a spot on the drivers side running board bracket to mount the under-cab gas tank. If you are using the 56 gas tank that is in the cab or an aftermarket under-bed tank then this will not be a factor either.
Thanks all. The gas tank will be relocated under the bed and I will go with a coilover springs in the rear so the frame should work just fine. Thanks again.
Ray