Scavenging and installing a more modern rear for a few hundred dollars?
Transmission
Rear end
Drive shaft & u-joints
Springs, bushings, bolts shackles
Various mountings
Replaced bearings, bushings, and seals
Labor
A few hundred dollars.
Compare the above costs to the costs you listed for the 3.55 conversion with bearings, bushings, and seals.
I can see distinct advantages to upgrades to the original drive train. But, it is hard to see a significant cost advantage.
