backing plate?

The backing plates need to match the drums you put on. If you have sbp backing plates then they will go with your sbp drums.
If you go through Dr diff he will sell you the full assembly and it will all work together.
If you DIDNT go through Dr diff, then you would need a full drum brake assembly from any other mopar 8.75 and that would work as well but you just need to make sure you match THAT particular backing plate to its respective drum.
They all have their own offsets to work with their particular drum and hardware