I have a 03 Cobra as well...did not upgrade...but measuring, I am looking for the same specs...Possibly 19'' Rims, but in the same config...9s in front 10s in back.
18-10s will fit front or rear. If you go 10.5 in the rear you may need to clear the bolt head or run a spacer. I run 18-10.5 front and rear with 305-30-18s front and rear. I space them out a little to clear my coilovers. 18 inch rims will give you a little better handling because of the shorter sidewall. You will get a little rougher ride. Dont go 19. 18 inch is much better as far as tire selection and price.
Does this hold true for the 99 Cobras? Could one get a 17x10 or 18x10 wheel for the rear (using the buttonhead bolts) and squeeze a 315mm wide tire under there?
Ask Uncle Meat. He should know. I remember a long time ago he mentioned something about getting a smooth IRS bolt to clear tires/wheels. Worse case scenario, your could get extended lugs and a wheel spacer.