Q1. Power assist in brakes doesn't work or works only intermittently at low engine speeds. Main hose appears to be in good shape. Can unit be rebuild or must it be replaced. Please recommend sources to buy. Any other possible cause of this problems.
A1: There should be a one-way valve somewhere along the way (perhaps in the booster itself?). The engine only produces vacuum at low throttle settings so the one way valve keeps the 'peak' vaccum in the booster, instead of the current possibly not so strong vacuum. Its a separate gizmo inline on my 1800E.

Q2. Loud noise/vibration above 45 MPH...early referred to as "rumble" by someone else. Probably U-joints and/or carrier bearing. Will investigate this weekend. Any other possible causes?
A2: Probably the above - vibration is usually the first sign of a bad u=joint, followed eventually by audible klunking when going from 'push' to 'pull'. They usually don't rumble that much, however, in my experience. Center support bearings will certainlly rumble plenty, but usually not vibrate too much. Could also be a bad wheel bearing. Or faintly possibly a bad differential, but they usually howl gear noises before bothering to rumble. Chock the front wheels and jack both rears up and set them on jack stands. Then turn the drivetrain by hand and carefully listen for bearing noises (a quiet garage is a help here). If you can't find it now just wait a couple of thousand miles until it becomes more obvious.

Q3. Some minor "free play" in steering. Otherwise, the car steers and tracks fine. Ball joints and tie rod ends all feel sound. Can the steering box be adjusted to eliminate this? Any other bushings I should look for, that may be worn?
A3: There is an adjsutment on top of the box - a lock nut securing a threaded rod with a slot in it. To adjust get the wheels pointing STRAIGHT ahead - then loosen the locking nut and loosely screw the bolt inwards to remove slack. This shouldn't be snug, but close. If you get it too tight (or adjust it with the wheels turned where there is normally more slack) the box will quickly self destruct. While you are paying attention to the box take off the plug and see if it wants any gear oil. It probably hasn't been checked in 20 years.