[MikeP]

You can SVA the car with an old engine block and use the casting number to prove it's age. The code is very simple. The casting no. is under the ledge at the back of the engine on the right hand side. The first letter is the decade C is sixties; D is seventies. The next number is the year. eg C7 is sixty seven. You can get the info from most small block building books to provide evidence of the age of engine. Pick a year which requires only that it produces no visible smoke.
You can get any plate from your SVA process and then put on a plate of your choosing provided it does not try to make your car look younger. Making it look older is not a problem. I am putting a D suffix plate on my GTD (which is a 1966 plate). The car is 1989.
Using an old logbook from an old car to register your car is more difficult as the logbook will say something different than a GT40. Also suitable logbooks (which have details which are something similar to what you want) are not easily obtained and not cheap as they are in demand from hotrod builders. The method is now complicated by the MoT sytem being computerised and hence hard to change details.
Technically you should use the horrible yellow plates on a car which is fairly modern irrespective of the plate however I do not think that there would be an issue on a classic type of car like a GT40. I shall be using the pressed metal plates on the rear and either stick on or raised plastic letters on a fine black mesh at the front.
Cheers
Mike