Andy, someone else shoud be able to confirm fitting on the GT4, but my thoughts would be these:
1. Measure clearances currently available on inside (to suspension bits) and outside (to guards) of existing tyre/rim setup;
2. Calculate rolling diameter of existing tyres, based on nominal size / aspect ratio / rim diam;
3. Decide if you want to increase or decrease overall diameter of new tyres for gearing purposes, esp as using on the track;
4. Depending on 3. above, the clearances avalable to existing bodywork, etc may need to be adjusted;
5. Compare wheel offsets of all existing and new wheels. Any offset increase will push new tyre outwards and reduce outer clearance avalable and increase inner clearance available (for same tyre width);
6. Calculate widest tyre section able to be fitted at each end by adding avalable inner & outer clearances to existing tyre width, less running clearance (say 1" all round?);
7. You have now determined the max tyre section width that can be fitted at each end. Working backwards from the overall diameter required (point 3), determine the tyre aspect ratio (eg 55%) that gives this diameter on those rims;
8. Now check the rim-width / tyre-width relationship. Are the tyres too wide/narrow for the rims? Ideally the rims should be the max recommended for the tyres, not vice-versa, as this will give best sidewall stiffness and steering response. Check the tyre specifications of your calculated sizes from a couple of manufacturers, looking for what rim widths are recommended. Hopefully you will be higher end.
9. Bear in mind that higher/wider tyres will have greater unsprung weight and rotational inertia, neither of which are very desirable.
(Sorry about the ramble