If you ask me, dual-pivots on the rear are a bad idea 99% of the time. They're too powerful to allow for good modulation when the wheel can lock up so easily.

I'd just keep using the old rear brake. A DP on the front is both necessary and sufficient.

That said, assuming you want to use the new brake anyway, instead of mounting it on a flexy bit of thin plate with a goofy-looking spacer making up for the long spindle, I'd mount the brake in the upper position and use a V-brake noodle or two to allow the cable to run along the seat tube for a bit and enter the brake from above as intended.



V-brake noodles are the answer to many a cable-routing nightmare. They can be bent or shortened if necessary.

I had to shorten one recently to fit V-brakes to a bike that only had that little s-shaped tube around the seat tube for cantis... I filed the end of the tube more square and used an old-style acorn to mate a short length of housing to it, running to a shortened noodle.