What's the best way to measure for tire size.

Well first, a 6" offset on an 18x9 isn't a 0 offset, it's a +25mm offset. Backspace and offset measurements are tied together, you can't change one without changing the other.

The '94-04 Mustang wheel is what you would be looking for, they have an 18x9 with a 5.95" backspace (+24mm) offset. Remember that with the Bullitt rims you have to open up the center register a bit to get them to fit over the hub on the rotor in the front.
18" Anthracite Bullitt Wheel 18x9 Rim Fits 94-04 Mustang® GT V8 V6 CP | eBay