As for a rear tire size differential wearing out the LSD clutches, that's exactly what happens. I have yet to find ANY spare that even comes close to the OD of a 285 or 295-50 x 15" tire. And if the worst happens and you get a flat rear (mine was a suddenly porous Campy rear wheel- so an inflator can w/sealant was useless), transfer a front tire & wheel to the back, put the spare on the front and observe the 50 mph@50 miles distance limits for spares. Or try a cell phone, relax & hope for the best.
If you have a high pressure spare, maybe carry a tire inflator- either a can, or if that doesn't pump it up to the 55-60 psi you need, use a simple hose & gauge that plugs onto an adapter in one spark plug hole with the engine idling. Your ZF clutches WILL wear during the drive but maybe not as bad as they would with whatever spare you carry. And the front tire will better carry the 58% weight back there.