I just read the thread and saw the pic in your last post. Are you sure which axle you have? a 340 should have 8.75. If so, then axle shaft swap is easy. I got my BBP axles from Moser Engineering fo my swap. http://www.moserengineering.com/ .
They have several perf levels , I got their street axles. They probably have a set ready to ship.
It was an original 318 car, how can I tell for sure which rear I have, I was told it was a 7.25, but they could have been wrong, I will take a pic of it and post later, but if there is some other telltale way, please, let me know, thx.