You could get the JL8 disk brakes from the factory or over the counter as a service package. There is a guy near me who is restoring a JL8 Z28 whose owner ordered the rear disk brakes by mistake. When they asked him if he wanted disk brakes he said sure. He thought they meant front disk brakes. He didn't want to pay the $600 extra so they gave it to him for half. He got a factory JL8 by mistake!
'69 RS SS L78 convertible, M22, 3.31 posi, deluxe interior, gauges, rosewood wheel, AM-FM,fold down seat, SS wheels