This is a great site. You will have a lot of fun trying various combinations out.
I have found this helpful. They have a tyre sizing guide where you can enter all widths, heights etc.
Comparison are given.
www.discounttire.com - go through the
Info Centre part of the site at the top then go down to the
Tyre Calculator.
You can enter the current tyres on the Cobra and then options you are considering. The results will show you the various heights/diameters etc.
Considering tyres try
www.summitracing.com - as well as they have some good products.
Import cost is about $30 through a Pomona Cal. based exporter to Australia.