I had 25yrs working in and running bodyshops in the UK up untill a couple of yrs ago and have carried out many shell changes.

The bodyshops don't stamp up new shells and niether do they get involved with DVLA. Nor do the insurance companies.

If a vehicle is reshelled with a secondhand shell then they must be inspected and a new no. issued which a garage will stamp around the turet to or on the right hand inner wing and will get a invoice or cover letter to say why it has been done.
The original no. should be XXXXXX out but still be readable so anybody inspecting the car can still read the no. and determine where the shell came from.

With a NEW shell NO ATTEMPT should be made to stamp or weld anything into the floor.

Some advice I was given when haveing a reshell inspected once by the police was to remove the original no. from the old shell, Trim it nice and neat and fix to the floor with moulding tape.

That way anybody needing the no. can view it through the carpet (for ordering parts etc)
But anybody can peel it off and check the shell was new and untampered with (ie THE POLICE)

If you do not fancy this still remove the no. from your old shell because if you sell it, or some unscruples person gets it they can try to resurrect the I.D from the no and cause loads of grief for you.

I also use to work for a large insurance companies recomended repairers and would get there stolen and recovered vehicles in to RE- RING back to there original I.Ds so they could put them out to auction to re-coupe some of there payout.
Different insurance companies have different policies in this area hence you get some recovered vehicles going as spares only.

Hope this is of some help to you or just given you a few things to consider.