B body rear brakes onto A body axle

Remember that when you increase the brakeshoe width on the rear axle, there's more brake contact to the drums and the brakebias will also shift to the rear.
Meaning the stock proportioning valve will not meter the correct amount of fluidpressure anymore and overbraking on the rear wheels will occur.
If not delt with properly, one will find out the hard way in a panic stop or worse on wet roads.

An adjustable proportioning valve will be necessary.

I telling this because I've installed 2.5" wide, 10" diameter drumbrakes on my Dart myself, and even with the 11.75" C-body discrotors and large 2.75" calipers up front, the rear brakes still want to lock up during panic stops and more easily on wet roads.