I would recommend a MOT after a build as it helps with the DVLA V5 document changes.
The DVLA look up-on a MOT as an independent inspection confirming the car exists, it's in road worthy condition, it's the correct colour, and if the MOT tester does his job properly he should also change the VOSA details on record to read body type "sports", thus also confirming the body change.
It also confirms that you have built your car to a road worthy standard.
I recently MOTed a newly built car only to be told by the MOT tester that one of the wheel spinners was lose!!! And I'd been around the Industrial estate a few times before hand.
All that said a customer has just (last month) re-registered a SAMMIO SPYDER under the new TAX and MOT exempt rules!!
He just sent photos, body receipts, V627/1 and a V112.
The car is now registered as a Triumph Spyder .
Note- - IMHO, I also think keeping the car's original make helps please the DVLA, it might help with insurance and might also help registering it aboard, so I recommended doing so.
Paul L I also agree with you on this issue.
jaguartvr A good HPi check might give you some information on the modifications dates.