11" Backing Plate Differences

Typically the backing plate and drum and axle all have to agree.

If the backing plates are for an OTHER than A body then they are in agreement with OTHER than A body axles.

Then they have to be in agreement with the width of the drum.

A 2" depth backing plate will rub a 2.5 drum, a 2.5 depth backing plat will not rub a 2" drum but the shoes will not be on the correct part of the drum and might not be making contact on the inside nearest the backing plate.

What I would do is measure from the outer face of the axle flange to the lip of the backing plate and then look at drums for the other widths available.

There might be 2, 2.25, 2.5, 2.75, 3"
This all assumes that you have stock mismatched components.

Good luck.

It's not impossible just time consuming.