I've got the '82 DOHC twin pistons on my front end(78 750F). The calipers do not fit the 78 mounting brackets so I had to use the '82 brackets, otherwise a bolt on affair. I'm looking into putting the dual pistons on the rear as well. It looks like you would need the '82 bracket/spacer, caliper, and stabilizing rod to do the rear conversion.
I have no idea if it would fit correctly and have only been going on the assumption that the twin piston caliper won't bolt on to the 77-78 rear bracket.