NeilF355 – Sorry this is a quick reply as I’m supposed to be going to B&Q!
I believe historic is a rolling 40 years at the start of the year (i.e. 1st of January).
E.g. My April 1980 car will not become historic until 1st Jan. 2021.
Also, as far as I know, you could keep the donor registration post SVA/IVA provided you used two major components.
So even if the V5C says registered in 2007, it would be OK to use the original 1973 M plate.
However, if the DVLA records say it is a ‘new’ car ‘born’ is 2007 it can not claim to be historic until 2048!
Cheers, Paul.