Hey there,
I'm currently re-doing my 67 GT Fastback into a full on, one-off pro touring custom for sema 2011 (if all goes well) I went with Rushforth Wildside 18x10's for the back with a 295/35/18. On the back of a 67 they look huge! I would say based on the different tire manufacture's and the research I've done (diff Manufacturers may fit bigger/smaller) a 315/35/18 is the biggest you can go without getting into the frame rail, would which result in interior panel modification: this would be all the stars lining up, nailing the backspacing etc. I actually got a chance to meet both Ring brothers at Sema this past year and asked them how come they don't run a 335 or 345 on the mustangs they build like everyone and their dog run on the first gen camaro's. They said because size wise those big tires don't look right and also, to get into the frame rails and re-working them and all the interior panels is a ton of work, that makes the interior of the car a lot narrower than it already is..
A visual trick to make for a larger appearing wheel, is to order with a small amount of backspacing which creates the dished look. I went with an 18x10 on the back with 4.75" of backspacing - this resulted in a little over a 4" dish in the wheel.
Check out my website with some updated pics of my new wheels. I have the complete air ride suspension kit front and rear and once I can get the wheel tubs done, I will be ordering the custom dutchman rear end and will hope to have the ride height set this spring..
Good luck!
http://www.cardomain.com/ride/359421