Basically this works on a points system, the various parts of the car are 'worth' points You need a total of 8 points to avoid the IVA, the chassis is 5 points from memory, look up the points scheme, there are plenty of links.
ACE site probably provides the best explanation I have found, and they clearly explain what you can and can't modify about the chassis, which bits you can cut off, and which bits must remain untouched.
http://www.the-ace.org.uk
Provided you keep most of the original parts including the chassis and brakes/suspension/engine/transmission you don't need an IVA, You do need to register the change of body though, the guys here can guide you through that.
The important thing is to provide photographic evidence for before/after of the change of body. You still have to comply with the 'construction and use regs' relevant to the age of the chassis/first registration, particularly regarding the emissions etc.
All that said, in my opinion even though you may not need an IVA, it's still good practice to at least try to get as close to the IVA requirements as your finances and the design of the car allow.