The the transmission on my '73 shifts nicely through all gears except 5th. What the heck would cause 5th to grind? linkage need adjustment? Time for a rebuild?
What? Where do I start?

Start with checking/replacing all the shift bushings and adjustment

You have to do this methodically. First, make sure all bushings and linkages are functional and properly installed. Then you need to make sure everything is adjusted within it's proper range.

Lots of threads on this out there. Here are a couple:

Bushing replacement, in general:
http://www.914world.com/specs/fwsb_replace.php

Shift linkage adjustment:
http://www.914world.com/bbs2/index.php?showtopic=188120

another,
http://www.914world.com/bbs2/index.php?showtopic=91028

Use whatever procedure (Dr. Evil's, or Chris') works for you. Reading through them helps understand how everything is supposed to work. Only after checking all that should you consider tearing into the gearbox.

I would add that a GOTCHA in the linkage is the midpoint Conical-Bolt. This one is very tricky, torque, jostle, torque, jostle, torque, jostle, torque until you are sure the two parts are solid and the Bolt is seated, small movement within this part of the linkage is a very frustrating bug in the system. Might lead to a tranny tear down when you do not need one.

Ask me how I know.