firstly i would go for a rally shell as although it should be strengthened almost guarantee it has had a few knocks and backs along the way especially if it was forest spec. also a rally shell should already have its own identity as rallies require a full mot and a log book. we have a works shell and they were all LHD for fron diff sizes. and used that to build up from and is now on a q plate. as for going the SVA route it would be a kit car and therefore prob high insurance etc but you will get a plate relevant to the year it passed the test. ie if was done now would be on a modern style 61 plate
So if I built a kit car, using a second hand, unregistered, chassis, second hand engine, suspension etc etc, it would now go onto a brand new plate, even if the bits were 20 years old? That seems a bit odd!