A-body spring perch centers are 43"

my A-body 8 3/4 housing is 52 1/2" wide IIRC

best to measure from flange to flange

the backset on the backing plates will vary with brake and shoe size/width

are you looking at 8 3/4 or 7 1/4-8 1/4

early A-body or 73-up

stock A-body 8 3/4 had 1 3/4" brake shoes and small wheel bolt pattern

i used moser axle with the large brake offset for B-body brakes in my 68 Dart 11x2.5