I've got the bigger drums and booster on my ute.
I fitted the different back plates and the larger 6 cylinder 12 x 3" drums plus a booster to my 1972 SIII SWB ute. It stops as good as the Defender now. Before the fix it was like the brakes on a kid's tricycle. New drums are fairly cheap. Mine came from Brit Parts, Perth and the back plates came off a bush relic with the permission of the owner who was using it as a shed. The wheel cylinders are different on the bigger units; larger volume. In my opinion this is the absolute best modification that you can make to the early brakes.
disc brakes for series 2A
I have seen adverts in the UK land rover magazines. I also think that there was an Australian source.
Nick