You definitely have to remove the steering wheel in order to remove the binnacle, but before you do anything disconnect the battery and make sure that the steering wheel is in the straight ahead position. The horn push simply pulls out and once you have loosened the steering wheel clamp using a c spanner you will find that it is still retained by a simple c clip which needs prizing out. As John Keighley says the binnacle is held in place with two screws at the top and two at the bottom but NB they may be different lengths. The binnacle can be quite a tight fit and may need a little gentle persuasion to get off.
Once the binnacle is off I recommend that you remove the four screws that hold the instrument panel in place so that you can angle it forward to improve your access. You may need to remove the speedo drive.
If you are going to remove more than one instrument at a time don’t rely on your memory as to which wires go where and label all the wires you remove
Coming to the instruments themselves
The ammeter is of course wired as an ammeter and (almost) all the current to and from the battery passes through it. Replacing it with a voltmeter is not that straightforward as you have to maintain the circuit that it is in. Also bear in mind that a voltmeter will only tell you that something has gone wrong sometime after the battery has started to die. I’d suggest that if the ammeter is jiggling all the time it’s actually doing its job and is flagging up a problem in somewhere between the dynamo and the battery which of course also includes the return path via the vehicle and engine earth paths. This might be down to the connections on the back of the ammeter itself needing to be tightened but an ammeter is a very simple device and there is pretty much nothing that can go wrong with the internals.
The petrol gauge will, contary to what you might expect, show full, full, full all the time if the sender in the tank or indeed the wiring to the tank is open circuit. So that’s unlikely to be a problem at the tank end. I haven’t experienced your problem of showing empty all the time but I’d suspect one of two possibilities, either the supply to the gauge has failed or the earth on the gauge has failed. Once again the gauge itself is a fairly simple device - I found the best explanation of how it works on, of all places, an Austin 7 website. Essentially as I recall two electromagnets work against each other to operate the needle, which requires power from the battery to the gauge AND an earth on the gauge.
Which brings me to the next point, this sort of petrol gauge and indeed the mechanical temperature and oil pressure gauges do not need a voltage stabiliser, so there isn’t one. Which is a further complication to replacing the temperature gauge with an electrical gauge.
The clock originally relied on an electrical contact pulsed by the balance wheel which over time will have tarnished and pitted to the point where there is no contact and everything stops. As far as I know there is no clock that is the right size in anybodies catalogue any longer. I think there was an article in a fairly recent BOC Bulletin describing a modification kit that is available that substitutes an optical interrupter for the said contact, which, coupled with some clever electrical circuitry archives the same effect. I’ll try to find time to look it out.
While you have access to the back of the instrument panel I’d counsel replacing the extremely dim and inadequate instrument bulbs with LEDs. Why anybody ever needed a dimmer rheostat on those original bulbs is a mystery to me but the good news is that the existing dimmer rheostat will still dim the instrument lighting if you leave the old incandescent bulbs in the lighting over the central heating controls and switches. I used
https://www.bettercarlighting.co.uk/index.php? It’s also well worth putting an led in the interior light.
Presumably you are happy with the accuracy of the Speedo and the Rev Counter. But it is worth knowing that the Rev counter can get to read much higher than actual rpm over time. If you need to know more please come back to me
Roger