best way to move back rear axles

-And possibly re engineer the rear shackle angle.
-Dont forget those front hanger bolts are pretty puny. The lever action at the spring eye might be something to consider.
-if you redrill the springs(good luck with that), you need to consider axle wrap.
-Moving the perchs might be do-able
-The shocks might move into the tank though, and the shock-bushings may be stressed. And the bump stops may be off.
-How about trimming the front of the wheelwells.
-When I put 325s (about 28")on my S, I shimmed the hangers back 1/4 inch, and trimmed the front corners of the wheelwell/quarters, and the wheel lip at the top was trimmed. Then narrowed the rear end to fit.I mighta moved the axle back on the perchs too. Cant remember; it was a dozen or more years ago.
-Just how big a tire are you working with?