10" drum brake swap for BP change

I have a 1967 Barracuda with a 1969 (A Body) 8-3/4” (489 diff case) .
I picked up some axles from Dr. Diff with 5X4.5 BP (nice!)
Purchased some new (73’?) 10” drums with 5X4.5 BP holes, deep (2.5 depth),
along with a brake hardware kit and 10”x2.5” shoes from Classic Ind.
The drum fits to the existing back plate, but the shoes won’t come together at the top.
Do I need a different back plate? Adapter? What?
I don’t think I can still use the old 1.75” shoes and I don’t really want to.
Has anyone run into this?