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.htmlIt 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