I am curious as to where the extra length comes into play on the new body, this will dictate what has to be done on a frame to make it suitable for the new body to mount properly.
shifting rear suspension is more than just weld the tabs down a few inches, the frame was designed also for the upper mounting of the springs/shocks and, the triangulation of the frame is optimised for these mounting locations...shifting the lower areas without also addressing the upper areas will lead to a stressed structure or suspension misalignment.
maybe i am reading into it a bit deep, but i feel building a tube chassis, there is not much room for error, it is designed to self support and once you take something and alter its intended use...its anyones guess.
not to create stress or downplay the new body, i am just as interested in this as anyone, but again, this is just my opinion.