Best thing to do is call the DVLA for advise and tell them your intentions.
I know for a donor car, to have an age related plate you need two major components. See below section A
http://www.dvla.gov.uk/vehicles/regrebil.htm
Not sure about the Cat C bit though. Usually, to get a registration for a Cat C, it would need to be repaired and inspected before it's allowed a registration. But as your doing a kit car, not sure it this applies.