Sounds really interesting and it would be nice to see pics at some point !
There is a lot of discussion about this over on the Sammio/Miglia sections, just don't ask me exactly where!
There is a set of guidance notes somewhere from the DVLA that advises what you are allowed to do to a chassis before it's classed as an alteration. Again, my apologies but i can't find it atm, but i'm sure one of the others (Paul?) may know what i'm talking about and where it is?
From what i remember, there was 2 things that stood out to me about their guidance
1.) It basically said that on a separate chassis car such as these, all you're allowed to do is alter / add " supporting brackets"
2.) With a Herald, what exactly is "chassis" and what is just "supporting brackets" can be a bit of an interesting conversation.
i.e. do you count the side outriggers as chassis or body mounting framework/brackets? There're not there on a Spitfire and on that the core of the 'chassis' is largely the same as a Herald! Same goes for the rear outriggers / boot-mounting brackets!
Without seeing exactly what you've done then i'm not sure anyone can give you a 100% answer and even the above is just my opinion and i'm sure others think differently.
But to me as soon as you said "cut the chassis" then i'd say you're on very shaky ground!!
Dave