Best brake booster
So what would your advice be? I won't hold you to it.lol.
As I previously stated, Doctor Diff for most of the parts.
Bendix Style Dual Diaphragm Booster for Mopars (listed for '67 and up A bodies, but will fit your earlier one as well) $300.00
Mopar Front Brake Line Selector for A/B/E Body $45.00
Mopar 1 Piece Distribution Block $65.00
Your choice for master cylinder, depending on if you want a bolt-on lid or one with a wire bail (Rock Auto for reference):
1967 Valiant, 4 wheel drum brakes-
More Information for CENTRIC 13063009 $45.79
1973 Valiant, 4 wheel drum brakes-
More Information for CENTRIC 13063015 $58.79
So, for between $455.79 and $468.79 plus shipping, you'll have everything you need to add power brakes with a dual-reservoir master cylinder; using Mopar-based parts. You may be able to source similar master cylinders cheaper- I just used good quality mid-priced units as an example. The generic unit was at $449.99; so you're right in the same price range, with what I would consider to be better parts.
Contact Dr. Diff (contact info on his website) and tell him EXACTLY what you're planning to do, to make sure you order the correct pieces. He won't steer you wrong.
Just my opinion, for what it's worth. Since you asked. :)