Ben
The first thing to note that SVA and registering teh car are two separate processes.
Once you have passed the SVA test, there are various routes to obtaining a registration number. As Tony says, you can get either a new registration, or a Q plate.
However, there is third method ( which is the one I think you are referring to ) which involves presenting your car as a KIT CONVERSION ( not a Kit car ).
The rules for this are explained on the DVLA website - as follows:
This is where a kit of new parts is added to an existing vehicle or old parts are added to a kit comprising a manufactured body, chassis or monocoque bodyshell. The general appearance of the vehicle will change and result in a revised description on the registration certificate.
A vehicle will retain its donor registration mark if either the original unmodified chassis or unaltered monocoque bodyshell and two other major components are used. If a new monocoque bodyshell or chassis from a specialist kit manufacturer is used (or an altered chassis or bodyshell from an existing vehicle) together with two major components from a donor vehicle, an age related mark will be assigned. The mark will be based on the age of the donor vehicle. An ESVA, SVA or MSVA test will be required to register the vehicle.
Where there are insufficient parts from a donor vehicle or in cases where the original registration mark is unknown, an ESVA, SVA or MSVA certificate will be required to register the vehicle and a 'Q' prefix registration number will be allocated.
The Important and relevant words for GT40 builder are ' If a new monocoque bodyshell or chassis from a specialist kit manufacturer is used (or an altered chassis or bodyshell from an existing vehicle) together with two major components from a donor vehicle, an age related mark will be assigned. '
This means if you use, say, a 1990 'T'' reg Granada as the donor car forsay, the front and rear suspension and the driveshafts or steering, yuo should be able to convince the registration officer that you have used enough parts to avoid being given a Q plate.
The DVLA spells it out clearly. Just search on 'DVLA kit conversions' or 'DVLA radically altered'.
Good luck!