I THINK you might be able to get away with unbolting the brackets from the early poly and bolting them to the B-engine. I used the poly mounts and brackets on my LA. I know they're a little more closely related, but it just might work. Worth a shot with a dummy block. I believe the B-engine didn't show up until 57 or 58, so I am assuming that the mount/frame configuration changed enough that that would be the simplest solution to the problem.
If you want power brakes, look at an A-body unit. Basically a bolt-on and will get the MC up and out of the way. I know you never asked, but look at
www.aajbrakes.com for a reasonably priced disc conversion for the front.
As for the rear, I used a 70-74 E-body unit in mine. Based on the chart located here:
http://www.moparts.org/Tech/Archive/axle/17.htmlthe B-body unit should be about an inch-and-a-half narrower than the E-body unit, which was about ¼" perside wider than the OE unit I pulled out. I'd say either one would be fairly easy, requiring basically only the addition of the OE spring perches, if you want to retain the OE shock mounting setup.
