No responses yet.
Well here's what I have come up with.
Original tire was the equivalent of 225/70/14 (or there about)
That's 8.85 inches wide with a total diameter of 26.4 inches (though I have heard that the original tire/wheel combo came in at about 27 inches.) So we'll say give or take half inch.
With 17 inch wheels they recommend 225/55/17 for the same width and an overall diameter of 26.74 inches.
I have 275's on the back of the 2005 GTO and can't see going smaller than that on the 69 so I am thinking of this combo.
275/50/17 width of 10.82 in. overall diameter of 27.82 in. REAR
255/50/17 width of 10.03 in. overall diameter of 27.03 in. FRONT
Tires would be Nitto NT450 (only matched set I can find in these sizes)
I'd be within an inch of stock height all the way around and a bit more rubber to the road.
I know she has 275's on her right now in the back but they are 15's so I have no clue if this will work, and since I still don't know the original backspacing and offset on the stock 15's (or the crappy custom wheels I have now) I still have no idea where I am at in the fender well.