64 dart brake/suspension upgrade

Stay 100% Mopar. Use the '73-up A body spindles and brake hardware. The only problem you would have is if you had the early A body front sway bar. In which you would swap your spindles from one side to the other. You will here plenty of people groan about swapping the spindles from side to side but the concern is minimal.
that way you won't have the GM master cylinder in engine bay. If you want power brakes you can buy power booster & master cylinder as a complete set at auto parts store. You would have to find an A body power booster bracket.