Wilwood conversion problem

If you can find a set of rallyes to fit the front I'd probably just leave the rears alone for now and start saving to convert them over to BBP. Dr. Diff sells the axles for $300, and you can usually get loaded BBP drums from mothermoparspokaneparts on eBay for under $100. This set is 11x2.5, when they get 10x2.5" they're usually a little cheaper. The 11x2.5's need a 15" rim, FYI.

http://www.ebay.com/itm/11-2-5-PLATES-LOADED-MOPAR-DODGE-PLYMOUTH-8-3-4-DRUM-BRAKES-8-75-NEW-YORKER-/271819168063

There isn't an issue with running spacer/adapters in the back and not the front, but you'll probably run into the same issue with the backspace on the rim not being enough to run that spacer and still fit the wheels inside the quarters. Of course that depends A LOT on the backspace of the rear wheels and the size of the tire you want to run. But even the cast adaptors (which I don't like, seen too many cracked) are almost 3/4" thick. You'd be far better off going with billet, those are safe when properly torqued and maintained, but those are usually 1" thick. Usually that means an extra 1" of backspace will be needed on the rims, but again, it depends on what you're starting with.

Like I said, if it were me I would finish the front and run the 5x4" pattern out back until your funds allow you to do a proper conversion. That also has the added bonus of getting rid of the SBP drums, which are hard to find parts for, and replacing them with the BBP drums, which came on almost everything with wheels that Dodge made up until the early 90's.