It's tough to get things sorted out sometimes.
I think in general with these cars the front ball-joint-to-tire-sidewall clearance is going to be the biggest constraint, and will probably limit you to something around a 4.5in backspacing.
Offset and backspacing are not measured interchangeably. Offset is the distance between the wheel seating surface and the centerline of the rim. Backspacing is the distance between the wheel seating surface and the outside rear face of the rim. Rim width is measured from tire bead seat to tire bead seat.
So...you need to account for the thickness of the rim when converting from backspacing to offset. As a result, that 4.5in backspacing on an 8in rim width comes out right around 0 offset.
OEM 17x8 Mustang wheels including the Bullitt are 5.44in backspacing, the 17x9s like the '95 Cobra R style are 5.95in backspacing. Once you adjust for rim thickness these are both around +24mm offset (positive offset means the wheel is pushed INTO the fender relative to the hub mating surface.) Off the top of my head I'm not sure what the 18x9.5 '00 Cobra R wheels are.
To get from +24mm to 0 is why there's:
http://www.wheeladapters.com/
I'd advise using something with a hub pilot ring so that the wheel is hub-centric, not depending on the wheel studs to center the wheel.
Once some of the other projects are out of the way and I can get back to finishing the '64 wagon, I'm hoping for 18x8.5 and 245/45-18 in front, 18x9.5 and 275/40-18 in back.
<font size=-1>[ This Message was edited by: JEM on 1/4/04 12:50pm ]</font>