Most here will run a 255/60/15 steer 275/60/15 rear, 245/60/15-275/60/15, or 255/70/15-235/70/15 combo.
The rallies you have are probably 8" wide. GM didn't make 9" wheels. The 8" width is from the inside of the rim where the tire bead seals. I beleive the 8" rally wheels had either a 4 or 4.25" backspace.
Here's a shot of my truck with a 3.5/5 drop with 15x8 wheels with 3.75" bs running 255/60/15-275/60/15 tire combo.
The truck below is see ten's truck with 8" rallies and 275/60/15 tires all around and a 3.5/6 drop.