[Steve C.]

Generaly Firebirds can use up to 15 x 10 with 5 1/8-inch backspacing in the rear.
Backspacing... measure the distance from a wheel's mounting flange to the farthest point inboard on the wheel. Some manufactures measure from the mounting flange to the bead and not the little lip around the circumference of the rim.... check with the manufacture. Also make sure to measure from the exact point where the wheel will contact the car, not from any depression that may be around the lug nuts.
On my '70 TA I'm using 8.5 x 15 wheels with a 3 3/4-inch offset. For the street I'm using 275/60R15 Nitto (555R) Drag Radials with a 27.76 inflated diameter and a 11.1 overall width. I run them with 25 lbs (cold) air pressure.
I decided on the Nitto's because they get much better tread wear compared to other DOT style drag tires and unlike these other drag tires there is some salvation if you get caught in the rain. Dispite 589 ft.lbs. of torque they do a fair job on the street for spirited driving (I do not street race).
Remember when choosing radial or bias tires for the rear make sure you use the same type on the front. Do not mix bias and radial tires.