I do not think you need take off the steering wheel, though it might make access easier. I would be tempted to update to a voltmeter.
For information - Caerbont Automotive Instruments Ltd (CAI) is an independent company in South Wales who have their roots in the Smiths Motor Accessories business. In 1984 ownership moved to the Lucas Group and then to VDO before becoming independent following a management buyout in 1993. CAI was the subject of a second MBO ten years ago.
They have the majority of the original Smiths tooling and should be able to repair or replace most of the instruments. They have also bought up Speedy Cables. I have a full suite of 411 instruments on order from them - they are prepared to make bespoke ones if they have the pattern. For example I want the fuel filler symbol (words would be disproportionately expensive) to point to the side of the filler cap as most modern cars do.
Their email is at
info@caigauge.com
Brian