[edited because I realized it was confusing]

I have 10" narrow (1.75" width) drum brakes on my car now (66 a-body).

If I switch to 2.5" width brakes (drums, backing plates, etc), will the overall width of the rear-end stay the same?

Or will I need a different wheel offset?

It seems to me that if the wider drums increased the overall width of the rearend, your axles would be too short. I always assumed that's why there were different backing plates. Are you running SBP axles? Do they make 10 x 2.5" dums for them? Never looked into it, is why I ask.

Correct me if I'm wrong: you have BBP 8&3/4 aftermarket axles made for the SBP 1&3/4 shoes/SBP backing plates? these axles have a different "standout" than A body 8&3/4 axles made for the standard BBP 10" or 11" brakes. If this is correct so far, you cannot use the BBP (2.5) brakes (shoes/backing plates). you would need to use another set of SBP drums & redrill them for BBP like you originally did & open up the center register (center hole) so it will fit on the BBP axles. I can post the drill pattern.

I think I once upon a time I had the correct custom Moser axles, in an earlier configuration, but sold them on, then re-ordered what I thought was the same thing from Moser when I redid the rear - I wonder if I forgot to order the smaller center register, so yeah if the axles are BBP and have the large register it would explain the mismatch. I think I bought a spare set of drums in the correct size, so I should just get them drilled, and open up the center hole. Weird that I didn't notice when I put them together (drove them like that for 15 years).

Her's how to run 10" brakes on an A body.
Get the F&R brakes and spindles you want off a '69-70 B body.
F- Bolt spindle and brakes on.
R- Axle is offset, get correct aftermarket axles and then bolt the brakes on.