Wotcha matey,
Before i built my Sammio i had bought a mk2 MR2 to convert into a Lambo so it's interesting to hear what you're planning! I ended up scrapping my project due to how much it would end up costing, as i went into it quite naive and Lambo bits aren't exactly cheap
It sounds like what you're planning will fall under the same rules as the F355 rebodies and the Sammio rebodies as in both of those cases you don't alter the underlying chassis and the vehicle is simply classed as a rebody / radically-altered-vehicle as long as certain criteria are met. There is some interesting reading and links to the relevant DVLA sections that in this thread:
http://www.madabout-kitcars.com/foru...ead.php?t=3280
As soon as you modify the "chassis" which for the MR2 we're talking about the monocoque, then you'll almost certainly need an IVA and then it's a whole different story.
e.g. for separate chassis cars we are only allowed to add minor non-structural brackets before it is no longer classed as radically altered and becomes a kit-car, anything deemed too major or altering the original chassis or affecting the main structural/drivetrain components of the original chassis then takes it in to the realms of IVA. There was a long discussion about this somewhere along with the DVLA/VOSA feedback about what is and isn't acceptable, but i can't find it atm. Sorry.
Also what is classed as part of the "chassis" is also a very delicate area for monocoque cars and the only guaranteed way to ensure that you don't fall foul of the inspectors is to query them and get them to clarify what is and isn't 'chassis' for the make and model of particular car you're using. But even though you're meant to be able to request that, i've not personally come across anyone who's done it, and from the joint experiences on here with DVLA/VOSA, they do tend to err on the side of caution!
TBH, as long as you stick with just designing new panels that bolt on in place of the old ones and you leave the monocoque alone, then you'll be fine.
I hope this is a good starter for 10? And i wish you all the best with your project!
Dave