Difference between the Fly Porsche 911 and the Monogram Porsche 904. There are not only big differences in the width of the cars, there are big differences in the width of the stock tires. Having said that, you can get a 3D chassis for the Monogram Porsche 904 that will allow you to use hubless 10mm width wheels and tires. 
I think the best way to avoid the tire and car width issue is to mandate a tire width. Find a tire-wheel combination that works and mandate that. You can also adjust the rear track on most cars to bring them all very close to the same rear width. The wider bodies will look really funny with the wheels pushed all the way in but that is a way to level the playing field.
I prefer to limit the cars by having mandatory parts requirements more than a random calculation to level the performance of cars.
Marc – you are the expert with these smaller “build” cars and are better equipped than the rest of us to suggest a specific wheel/tire make, model and size. Throw something out and let’s see how it works.