Like I said, if it is welded in properly then there would never be any way of telling that it had been welded in
If you don't think they'd be willing to go to the lengths of using a microscope to check the grain pattern of the steel around any suspected welds in order to prove you've avoided paying them years of road tax you are quite mistaken.
There is a system for re-stamping in this country. I looked into it many years ago when I reshelled my car with a Heritage body and I'll see if I can dig up any paperwork from it. There are problems with it, mainly in that most of the DVLA don't know the system exists or how to go about doing it. Most times if you ask the DVLA the person speaking to you will tell you that you need to go to a main dealer, as in this country the manufacturer is allowed to re-stamp. Main dealers are not the manufacturer however, the DVLA call handlers just think they are. The dealers don't have the right stamps, and are not willing to get a set for you. In theory they can go to the manufacturer with relevant proof and get permission and order stamps but they don't want to get involved. Really you have to go to a DVLA garage, the places that do VIC checks and so on. They can do it, and have stamps and things, as they are the places you go to in order to get a new number stamped if the DVLA tell you that you have to change the number for any of their obscure reasons. You have to have proofs and you have to have a letter from DVLA allowing them to re-stamp it and that is the hard part to get because as I said, hardly anyone there knows the system exists.