I have a 56 w/ a 350 small block & a 400 tranny. I put a 3.08 posi rear end in mine so I could cruise at 65 running about 2200 rpm's. You'll want to consider tire size as well. I found that Randy's Ring & Pinion web site to be very helpful. Their calculator tab at the top will assist you with determining what you might want. That said, it won't tell you which rear end you need, but it will start you on the right path.

http://www.ringpinion.com/Default.aspx

Paul
1956 Chevy Task Force Webshots
My wife puts knots on my head faster than I can rub 'em. That's why I ask knot-headed questions.