rnunn:
as Tom T. stated, your best bet is to look closely at the cage that ERAs come with, see Bob P. The ERA cage that houses the differential is about as tight and clean as it gets. I built my own rear end for my car, and I am very pleased with how the cage worked out.
Here is a link at ERAs site, that gives a diagram.
http://www.erareplicas.com/427/rsusp.htm