If you order a dual brake master cylinder for a '74 Dodge Coronet you will be happy!
I installed one on my '63 Stude Avanti approx 20 years ago, it's still working well, and brake performance could not be better (without updating the disc brakes themselves.)
Anyone still using the factory single piston master cylinder, on anything other than a seldom driven show car, is putting themselves and their Avanti at unnecessary risk