I know my 72 duster came factory with the small style booster ( like pictured above) and then in 73 and 74 I think they went to a mid sized booster and then 75 and 76 the started using a big ole booster lol.
so what you would be looking for is a 72 and earlier booster for a duster,demon,dart,scamp ect...ect...
only problem is......they are few and far between and the local parts stores stopped selling them (most of the websites still show them as available but the remanufacturer that supplies them stopped producing them) I do see them on ebay from time to time or in high end auto parts stores but unless you like spending $500 or so on a booster I suggest you hold out lol.
I searched for one for my duster for months....and after purchasing 2 used ones that ended up failing I gave up and converted to manual brakes.
you can get kits through summitracing for around $200.0 now I think but im not positive on that one.
good luck anyhow!