From what I was told by the DVLA pre sva builds go by the age related plate for the 40yrs , my 2+2 is registered 1978 and I was told that to get the status changed you filled in that section of the logbook and returned that part when taxing to the post office who would issue a "nil" tax receipt and send off the logbook to be changed.As you are in the post office you needed to take insurance and MOT paperwork.
The MOT exemption was a bit more tricky, the time starts from when the car is built into a quantum . But as some logbooks from pre sva days don't have a build/re registration date on them that's a bit grey.
Personally I think it's better to MOT anyway , a second pair of eyes as well as the testing equipment I think is worth the money.
Darren