I see where you are comming from but so long as you reafix the chassis plate to the new body tub it effectivley replaces the old tub. This would be entirely possible because the original tub would be in an irepearable state anyway and would naturally be destroyed.hence no rick of there being 2 with the same number. I build a new XJ12 Coupe some years ago with a new factory monocoque and all I did was precisely this the only legal requirement was that I provide proof of the destruction of the old body shell.