I just looked at the DVLA site page re MOT and Tax for historic vehicles.
https://www.gov.uk/historic-vehicles
The MOT section is clear re 40 years. Also a 30 year rule re no significant modifications, though this might need clarification to check for individual circumstances.
The Tax one gives an example that shows 40 years but not a clear statement re a rolling 40 years.
Here's a copy of a statement in the Historic Vehicle section....

You can apply to stop paying for vehicle tax from 1 April 2024 if your vehicle was built before 1 January 1984. You must tax your vehicle even if you do not have to pay.

I read that as January 1, 1984 as the qualifying date for taxing from April 1 2024. 40 years exemption implied. Perhaps trying to tax as a Historic Vehicle might clarify this.Also, ask DVLA.

Plus Four MY23 Furka Rouge