welcome to the forum !
you need to look into the chassis swap from a DVLA point of view. It is reasonably easy to swap the chassis and engine and so on.
However, were you to swap the chassis, engine and gearbox, it is unlikely to retain enough points under the DVLA scheme to retain it's original identity and would be issued with a q plate registration.
also, might be worth looking into what they will need for the SVA prior to starting work. That way, you can make sure you have all the necessary paperwork in place.
If you do a search on here, there has been a fair bit written about this, and a few people have fell foul, and not been able to register their vehicle.
wish you well with this.
