re: SSME question
I've heard there are about 12 unused SSME. I don't think this includes those on the Shuttles.
After the SSME have reached the end of "life" NASA has them stored. So these still good but beyond ratings SSME as I understand it are then ones to be used on the HLV.
I googled for information on the production line of the SSME and didn't find anything exactly on the subject. Found info on the SRB assembly line and ETs...