To change the rear main you'd have to pull the tranny, clutch, and flywheel. Unless it's a substantial leak (like you're going through a quart of oil every 1000 miles), I think I'd forget about it. The odds are good that a new seal would leak as well, this is a common problem with 5.0's and difficult to fix for good. Something to do only if you have to replace the clutch anyway.
Dave