The chinese voltmeter modules (did you delete the URL?) i.e. LED 3-digit 0.28" I have use a ME6203 voltage regulator+ series '5819 Schottky diode S4.
Which gives a ballpark +36V/-40V transient rating, not tough enough for an older car IMHO.
The vehicle will have dirty, noisy power. It's the ignition system, it's the dynamo brushes arcing.
Capacitors can help (at the noise source) but transient overvoltages are always happening. I dislike adding a lot of parts to vintage vehicles.
I did read Dr. Hugo Holden's work with the the Bosch 30019/Lucas RB106. I don't know it's voltage limits, the MJ2955 is rated 60V which is likely the limit for transient tolerance.
If you want to run electronics, cell phone charger etc. in the car I would (at the cig lighter) put in a very big TVS to clamp surge voltages like
SM8A27 which is commonly used in automotive ECU's. Then you will not damage things. It clamps to around 27V max. Add capacitors - at the coil power and dynamo if possible.
For radio (mainly AM) reception, resistance spark plugs or wire are used. I think you have shielded wiring for that.
Check you have both grounds proper. Battery(-) high current to engine block for the starter motor/dynamo, as well as battery(-) to vehicle frame/chassis. Two wires. I wonder what the voltage regulator ground point is. If it's on the firewall, then that is not the engine block, you get a different potential depending on lighting loads (headlights, brake lights) as they are grounded to the chassis. Electrical noise could also make the voltage regulator act a bit weird. I'd have to find a Bosch schematic but assume it's got filtering.