ok to convert 10 x 1 3/4 drums to 11 x 2 1/2 on '72 dart?

They don't have any for sale right now, but this seller usually lists their loaded backing plate sets for under a $100. I've bought a couple of sets from them.

http://m.ebay.com/sch/mothermoparspokaneparts/m.html?isRefine=true