Bikerjohn,
What sort of rallying do you intend to do? If you're thinking of stage rallys, single or multi venue, then you need to be aware that the RACMSA in their wisdom are altering the size and type of engines allowed.
So from 2012 max cc is 3000cc and no more than 6 cylinders ( which just keeps the Metro 6R4 boys in) and means you wont get a log book for a Rover V8.
I'm not sure how/if this effects the Historic rally cars, I can't believe the MSA would stop the TR7V8's but its certainly worth looking into.