check the big bolts forum we have had many discussions about rims and tires in there.

my understanding is that the 53+ rears could be swapped with rims off of a P-body truck. I have a 52 rear in mine and that won't work (or so I've been told) apparently the hub is to small and would need a custom spacer to fit the larger later rear rims.

you should be able to find the tires at big truck places. and they will probably do the work if you ask around.