3 down is the accepted normal for a daily driver, with more down being for more aggressive driving.
Lets say the owner will normally drive the car pretty easy.
But when he gets into it more the rear might see 2-3 degrees up under that load.
It will still be ok at that angle.
I don't remember, but it's printed on the shipping label.
You are correct that it is a GM caliper.
I probably would go for 5 on your car, because it's getting built to run.:D