What size tire are you planning to run? I f you want it to fit right without spacers or surprises (narrowing) you need to know your wheel & tire size first.
You can play with rearend widths and wheel/tire sizes with this calculator to get an idea, you will have to measure and plug your numbers into it.
A 10 inch wheel with a tire that fits it is going to be fairly snug in a stepside BTW
http://tri-5.chevyrides.com/rear_tire.php