Issue Resolved for me
After some additional testing I found that the following "fix" worked for me, of course YMMV.
On my print server I created a new duplicate connection for one of our shared copiers (that we were not able to connect to with x64, SP1) and installed the latest PS3 drivers and shared it. I was able to install it on all PCs, including my problematic x64, SP1 boxes. After trying this, I went back and tried again to install the shared printers with PCL6 drivers and they all worked. Even after I removed the newly created PS3 shared printer on my print server, my x64 SP1 machines now continue to install these printers again without issue.
Good luck to the rest of you! Hope this helps others like it did me.