The MK3 is a lot longer. The rear clip is much longer on the MK3. I'm not sure about the front clip. Here are some pics of Mk3 and a Mk2 for reference. You may be able to work out the main differences by looking at the bodies in relation to the wheel locations. If you can get both body styles together side by side you could get accurate measurements. From the two I have worked on the KVA Mk1 and Mk3 spider and doors seem to be the same. Not sure about the KVA front clips but the rear clips are very different.
It may well be easier to get new panels than to spend ages modifying Mk3 to Mk1.
Cheers
Mike