Here are a couple of links that may be of use/help with the changes you are thinking about
:
http://www.the-ace.org.uk/rebodying-a-clarification/
http://www.the-ace.org.uk/chassis-an...-modification/
What you're suggesting with the rear end of the chassis is going to need to be investigated be on a make-by-make case. Although I'd advise you read up on it yourself as if you ask DVLA the reply you get will be completely dependant on who you ask!!
e.g. As far as the Herald/Vitesse chassis is concerned, there was a lot of investigation done a year or so ago about taking the end off of the rear outriggers (that used to be left on and then normally "fell off" when cars were on the road) and what we found was they are actually additional body mounting brackets/outriggers that are added to the main chassis for the Herald/Vitesse and they aren't there on the Spitfire. But there was a _lot_ of investigation around that and comparing to other models!
Sadly I can't find the big post that Paul and I wrote that covered this in more detail, sorry, as It may of been of interest.
So fingers crossed for you!