Welcome over! Top Fuel and never been before? I hope that we can keep the drama to a minimum and make you welcome.

RB swap in a 55-56? I'm sure it has, but haven't read of anyone posting all the info. Is the car currently a V8 or 6? The V8 might help, considering that the usual issues are the narrow compartment and the steering on the earlier flathead six models.
Haven't heard of any drop in mounts, but there is new stuff popping up every month, and lots of little one-man shops that create these things.
Most engine swaps rely on a rear sump oil pan from a truck, but it all depends on the chassis and engine placement. Sorry I can't be more specific, but I just haven't read about one of these. The rear end that I used and others have used in earlier stuff is the 60s B body 8.75.
Here is a good list of widths.
http://www.carnut.com/specs/rear.html

As has been said in other threads, the build is your own, but it does help to keep to a theme and do lots of research and planning before starting.