Just found this thread again

I didn't realise that welding in new floor pans and transmission tunnels was an issue. I thought that as long as the majority of the car (roof, chassis legs, pillars, windscreen, doors, tailgate etc) all stayed the same that would be enough to keep it on its original plate as long as the engine capacity was changed on the logbook.

I suppose just through ignorance and the fact I've never been involved in a conversion like this before. Every day's a school day.