Hmmm....... Well,the last time I re-shelled a car,I used a 'previously registered' shell. I re-built the car and transferred the vin plates and other trim code plates etc,to the 'new' shell. I cut out the piece of floor pan,with the original chassis number on it and kept that to one side,before I cut the old shell up.
Now, when I put the car back on the road,I got a notice from the police,to take the car to their workshops,for inspection. They went over the car with a fine toothcomb and said that apart from the 'wrong' chassis number,on the floor,you would never know that it was a 're-shell'. The only advice they gave,was to trim the piece of floor,from the old shell and either weld or pop rivet it over the number on the donor shell. After that,they were completely happy.
Hope this helps you .....

P.S. It might help if you keep purchase receipts and some photographic evidence,that you have carried out a 're-shell' job and for what reason. You should have no bother,that way.....