I really should not tell you since you never said whether you got to meet the girl who posed with your car for Muscle Car Sales Inc.'s web site pictures.
In answer to your question, no, no one has ever bought a B body and thought about using a larger rear tire. I am kidding! That is usually the first though of each buyer and it is unusual to buy a car where somebody has not already pushed the limits.
My 67 GTX had 8 by 15 inch wheels all the way around with 235's on the back and 215's on the front. The back tires rubbed the inside of the wheel wells as they were a standard backspace. It is a bitch to set up a car with wheels and tires that don't rub, and then do when you have four people in the car, something in the trunk or want to do some spirited driving. Give yourself a little leeway. 67 GTX rear wheel wells are stupid tight. I got rid of the wheels an went to 7 by 15's all around and had no issues.
You can get Mopar wheels made from original wheel centers in any width or custom backspace here. Buying custom backspace rears is the key to going really maximum wide in the rear without cutting on the wheel wells.
http://themopardude.com/New_Custom_2.html
Summit and Jegs both sell Magnum 500 replicas with centers that look just like the originals, but in limited sizes. Most go to a 15 inch. These are from Wheel Vintiques or Specialty Wheels.
Coker tires can sell you Magnum wheels and original type skinny "spin em up" tires that look period correct including redlines if you are looking for old school tires.
https://www.cokertire.com/wheels/muscle.html
Diamondback tires can sell you redlines too, but also give you the option of buying BF Goodrich radial TA's in any size they sell, shaving off the raised white letters and turning them into a redline. This is about the best option if you want redlines and a great driving and/or wide rear tires.
https://www.dbtires.com/
As far as what you can stuff under the rear fender on a 69 GTX, do a search. You will get a lot of different answers. Many of the previous posts have pictures of cars that give you an idea of the stance with a particular wheel and tire combination.
http://www.forbbodiesonly.com/mopar...ckspacing-and-tire-size.72924/#post-910076621
http://www.forbbodiesonly.com/moparforum/threads/tire-size-69-gtx.31444/
So what did you find out about the brunette? Don't tell me you had the car delivered to you sight unseen or in the purchase process you never asked about her. And no, we are not buying that that is your girlfriend! LOL