Question about backing plates

My brother in law dropped off an 8.75" rear, axles and 3.55 gears. He said these came from a truck. I was planning to replace my stock /6 rear with this but they do not have backing plates or drums. I went to the local parts store and stocked up on shoes and a new spring kit. Also had the drums resurfaced. I pulled down the stock rear with 9" drums, and was going to use that setup. Soon after I quickly realized that the bolt pattern on the backing plates was not going to fit on the 8.75 rear. :banghead:

My question is what size brakes are supposed to go on the 8.75 rear?
Here;s a pic of the gasket for the backing plate:
http://s1239.photobucket.com/user/slotto1/media/scamp/05Jun14/IMAG0910.jpg.html

My short term plan is to clean up the stock rear and put on the new brakes then use that until I can get the $ together for the correct setup for the beefier rear as I'm gonna need backing plates, shoes, springs and drums.

Thanks in advance
steve