The front wheels are 20x9 with a 4.5” backspacing and have a 3” lip. This is the most common backspacing for a C10.
The rear wheels are 20x12 with a 3.5 backspacing and have a 7” lip. You will need a 54-56” rear end. I sell the the Speedway Fab 9 rear end separately on my website and it’s available from 48” to 62” in 2” increments. I’m running the 54” rear end.