The longer cable might work, but you'd have to route it in a curve or loop. I think it would be very difficult to shorten the cable with the push-in mount, although it could be done.
Using a '51 backing plate is a good idea. Bendix brakes were used all the way up into the 60's or maybe even '70's. Possibly with just some minor differences. Like Kevinski said, you'd probably have to see them side-by-side to see if they'd work. Backing plates were riveted on to the rear axle housing in '51, so you may have some work getting a pair. There are lots of torque tube rear axles around from conversions like yours, so shouldn't be difficult to find.
