What kit are you using?
I believe the rear-most body mounts are the same on yours and my '86, if so, the tank needs to be dropped. Rear bumper will need to come out too.
If it's the energy suspension kit, ignore their directions for the rear-most mounts. (again this is if your '73 is setup like the '8x ones) Pull the stock mounts from the rear after you drop the tank, and just cut the one piece off that interferes with their mounts. If you use their directions, you end up with a nut inside the frame, which is impossible to reach without dropping the tank, and just makes for tightening or loosening hassle later on, since the stock piece in that location has nuts welded to it, which means you don't have to worry about getting a wrench on the inside of the frame rail to hold a nut in place when you tighten the bolts on the outside of the frame.
May not make sense now, but it will when you get to it. (and yet again, IF the '73 setup is the same as the later ones)