OK
First thing first, make the NOVA declaration as you would with a recently imported vehicle
This will tick a box for the DVLA later on
Secondly has the vehicle been registered and do you know its reg number
If yes then go online and do a vehicle check on the DVLA website. If it comes back with a record then it is on the database and all you have to do is request a new V5
If it is not on the database then you need to use the V765 scheme. You will need proof that ties the chassis number to the number claimed. The dvla are very specific about the evidence required. An old style log book or a certified copy from the original county registration ledger or old not certificates are about the only acceptable evidence. The V765 application has to be submitted and completed by an authorised club.
If its not on the database or never been registered then you need to go down the V55 route for an age related number. You will have to have the vehicle inspected by a approved club ( the DVLA has a list) and dating evidence produced, this is then submitted to dvla along with the fee and completed V55 and eventually you should get an age related number
That lot is probably as clear as mud! However start the process before you strip the vehicle as for the inspection is has to be "reasonably complete"
If you want further clarification drop me a pm and I will try to help, I was MVT verification officer for a few years
Hope that helps