A question I should of asked before

Is this rear axle going into the Demon in your profile?
If so, there is no reason to narrow the axle to A-Body SBP specs.
Other than using the wheels that you have, you can get some BBP cop wheels to replace your SBP cop wheels and have the good brakes, big bolt pattern, and a source to a lot more inexpensive parts when you do this swap.
The SBP 7/16" studs are weak as opposed to the BBP 1/2" studs, so that is an upgrade as well.
All you need to install a BBP '68 to '70 B- Body 8 3/4 complete in a '70 to '76 A-Body Duster, Demon, or Dart Sport is to relocate the spring perches 1" total to fit. (That's 1/2" inboard on each side).
I have done this mod several times and you can fit a 255 60x15" tire in the wheel wells with a factory 6 1/2" wheel with ease.
This is what I would do if I were you.
And another suggestion is to change the front brakes to BBP '73 and up A-Body discs and you will be home free.

It seems like a lot of screwing around for now, but the benefits of the simplicity and the cheaper as well as readily available parts (as well as stronger and better) will pay dividends in the long run.
I also have non power brakes and used Dr.Diff's aluminum 15/16" master cylinder upgrade to have a lot better brakes.
I highly recommend this upgrade if applicable as well.

Maybe check out my build thread at the bottom of my post for more detailed pics and commentary for the whole swap if you are interested.

Thanks,
Tom.