Far as I know the issue is not with the inside diameter of the rim, rather it is clearance on the back of the face of the wheel. When I got my 17X8's custom made, they told me that I had to have 4 3/8" backspacing to clear the 13" calipers. (I had asked for 5" backspacing.
On some of the 17x9's, there is not enough backspacing to clear the big calipers. I believe that is what triggered GDiii to get custom wheels since the Cobra R's he had would not clear the big brakes without a spacer.
There are two different back spacings available on 17x9 Cobra R's. One I think is 4.75" and one is 4.25". I think the one is a Mustang offset and one is a Tbird offset. You need the Mustang (4.25") backspacing I think.
Hopefully I have muddied the waters enough that someone who actually knows will post and tell me I am right or wrong.......
As for rim width, that I do know about. The lower profile the tire, the less accomodating the tire is going to be regarding rim width. Also the acceptable rim width ranges listed on the tire manufacturer's listings don't always work out for best handling and/or wear. For example a 255/50/16 can go on a 7" rim according to the manufacture's listings. However, I can tell you from experience that most high performance 255/50's will wear really bad in the middle on a 7" rim due to casing distortion. 255/50/16's typically fit best on a 9.5" rim - the tire and rim combination that it was originally designed for - the Corvette.
Now not all tires are created equal! Even in the same size. I have sitting in my garage a 245/50/16 Pirelli tire that fits the 7" stock rim almost perfectly, and a 245/50/16 Firestone that is all bowed and messed up looking on the same rim. The Firestone is a FULL inch wider than the Pirelli! Using this same aproach I have some Michelin 255/50's on my stock rim and they look and perfect.
So getting to the point, on a 7.5" rim I see no reason why *most* 245/45/17's or even 255/45's wouldn't fit fine. However, it would be of benefit to compare different brands side by side. You will see the differences I am talking about. It is also usefull to check tire rack.com and go through the manufacture's specs and look at tread width's before choosing a tire. You do not want the tread width more than the rim width for good wear and handling characteristics.
Funny last note. Michelin does not provide the tread width specs for many of their tires. One of the reasons they do this is that if you have a high performance car, and you want the best possible traction, right? So you might shop for a wider footprint, right? They want you to trust them that even though Michelins tend to be narrower, they feel that they will handle just as well and ride better (what Michelins are known for!).
Another note: I originally purchased 245/45/17 Michelin's for my 17x8's. This is a perfect size match according to the books. The tire shop couldn't even get the tires to air up on the rims! They were just too narrow. So after trying for about 1/2 hour to get one tire to mount, we said screw it and put on 255/45's. Perfect fit! But like I said, Michelin's tend to be narrow, especially the Pilot XGT series.
Ok, I'm done now.