What do you mean by an over rated 318? I'm just curious since I don't know much about motors or why a 73 would be that much different than an earlier model. I'm assuming they could all be rebuilt similarly?
In '72 they dropped the compression on the 340, and about midway through the year switched to a Cast Crank (replacing the earlier Forged Crank). Also around the same time they switched from 2.02 Valves to 1.88 Valves.
I traded my Jeep Grand Wagoneer a while back for a freshly machined '69 340 Short block (not assembled, but all the parts are there), plus about $500 worth of A-Body and 340 Parts (it was mile long list of stuff)