Aftermarket A/C for '66, /6 Dart

Another possibility: buy or assemble a bracket setup for the old brick-shaped York/Tecumseh compressor. A variety of those were available for the Slant-6, and even if you can't find one any more, they're stone-simple to fabricate. Then toss a readily-available Sanden adaptor on it and probably sidestep the problems with the commercially-available aftermarket Sanden bracket setups.