First good luck with the sale of this rare and unique car.
Regarding disc brakes and the 1970 Plymouth Duster 340, the factory installed disc brakes as standard equipment on all 1970 Plymouth Duster 340s.
The MSRP for a 1970 Plymouth Duster 340 was $2,547 and included front disc brakes as standard equipment.
My brother was a car salesman at Bob Banning Chrysler-Plymouth-Dodge in Hyattsville MD, a close suburb of Wash DC, from 1968 - 1972.
I purchased new a 1970 Plymouth Duster 340 in May 1970, and indeed all 340 Dusters for 1970 came factory equipped with front disc brakes.
In 1971, Plymouth made disc brakes an option, not standard factory equipment, on the 1971 Plymouth Duster 340.
You can look around the web for the Plymouth Rapid Transit System advertisement, which featured all the 1970 Plymouth Hi-po cars for 1970, and that ad will lay out the exact factory standard equipment for the 1970 Plymouth Duster 340.
There are also plenty of car magazines, dealer brochures and the like which correctly document that factory front disc brakes were standard equipment on all 1970 Plymouth Duster 340s.
If you find a 1970 Plymouth Duster 340 without front discs, that will be the first, most likely you found a car that was either a /6 or 318 that has been modified to look like a tribute to the 1970 Duster 340.