To expand on what joekidd wrote -- The center "cap" insert when installed causes three stamped steel "legs" attached to the hubcap to grab a slot in the hub, thus securing the 5 pieces to the wheel. Once that center cap is pried out the hubcap can by pried off with a tool -- or your fingers.
My '80 245 with over 290k exhibits the same looseness in the steering column. Worn bushings inside the column is my guess---the play hasn't gotten worse in the 175k miles I've put on the car and doesn't seem to affect the ability to steer.
Check your temp gauge operation this way. Locate the sender -- it will be located under the front intake manifold branches - a single wire brass unit (the FI sender is 2 wire and located further back on the head)--it will have a yellow wire going to it. Turn on your ignition key - disconnect the wire from the sender -- ground the wire -- the gauge should show full H with the wire grounded. If it does the problem lies with the sender itself. If the gauge doesn't move with the wire grounded the problem is in the circuit or gauge itself. There's a gray junction block for engine wires - that would be the place to check first--then move up the line back to the gauge--that is--the wire going to the cluster connection--the temp compensation board (if it still has one). There are places on the BB that explain the comp. board and how to eliminate it. If it's the gauge itself -- it will remove from the cluster easily - I have lots of spare clusters available for parts.---Dave