Wheel/tire Fitmena and Offset question.

[OneStiffGT]

As the topic states, would 275/40/17's all around, on 27mm offset 17X9 wheels work on a 99 GT? The car is lowered, I've been told the right offset is 25mm's, but I am wondering if I can get away with a +27mm offset wheel.

EDIT : I want to add one more thing, I am running Cobra brakes all around, how much backspacing would I need so the wheels actually clear the calipers?

Thanks!