As the other guys metioned, those specs should be fine. However, I can say that for me I am not absolutely sure about the backspacing #. If you look on my sig, you'll see the tire sizes I have on my '79 and I would think if I'm doing fine with those, you should be able to at least get away with the same rim/tire combos. Let it be known, however, that I did have to roll my rear fender well lips (easy and harmless, and IMO, looks better than original) b/c the 275s would scrape the lip (almost cutting the side of the tire). Now it doesn't scrape AT ALL and everything is crisp.
With your 89 GT, you may not have to do this at all, only maybe worry about the quads...
Have you considered 15x9s for the back. I plan on getting those in the far future.
As far as the sticking out part, buy the rim you want (don't mount the tire on it yet-- cause if you do, and it is the wrong size, they won't let return the rim for refund) and without a tire on it, put it on the back and bolt it loosely with the lug nuts. See how it looks from a few different angles (from back of car, from side, from side front, etc). It should at the
most be flush with the fender well lip (even better if it is about 0.5-1" inside the fender instead of flush). Of course, if it extends