Question about backing plates

The rear my brother in law dropped off has no backing plates. It's just the housing, the gears and 2 sbp axles.

The car came with a /6 and small rear.
I sorta put the cart before the horse and bought all the hardware and got the 9" drums turned in hopes of transferring it all to the 8.75 rear. After I got all my parts together, I pulled the stock /6 rear and I noticed that the backing plates on the /6 rear has 4 bolts and the 8.75 rear requires a backing plate with 5 holes.

Is there a backing plate I can use for a 8.75 rear using a sbp axle for 9" brakes that has 5 holes?
Or would it be easier to try and return everything I bought and upgrade to the 10" drum setup for bbp?

sorry for all the confusion, I'm new to all this.
thanks,
Steve