If you are really stuck on keeping your drum spindles and booster, this is the only kit I know of off hand that will let you do that.

http://ecihotrodbrakes.com/dodge_plymouth_discbrake_conversions.html

It uses GM calipers that have a piston size compatible with the pressure a drum booster can generate.

Is there a reason you don't want to go with disc spindles and booster?

A-body spindles I would assume are getting scarce but there should be late B, R, and FMJ spindles that are easier to source.

Kevin