I got mine from Greg Sanford, he advertises them on E-bay under the name 2116cassandra
They are a converted truck rearend, so it is just a mater of how original you want it to be. He has alot of options as far as disk brakes and such also, I had him build it set up for Right Stuff disc brakes and everthing fit good.
Have heard nothing but good stuff about Dr. Diff also.