Over filling isn't too much of a problem, it will leak out the vent hose.
The vent hose is on totp of the tranny right behind the bellhousing. you have to reach up to find it, although on some models GM put a 8" length of black hose on it so it hangs lower.
If it were my truck and I drained the pan, I would add in 4 quarts. Fire up the engine and check the dipstick. Shut the engine off and add 1/4 quart at a time until you see it on the dipstick. Repeat until full. then take it for a 20 minute test drive and check it again, and check for leaks.
It takes 1/4 quart to go from the bottom of the cross hatch area on the dipstick to the top of the cross hatch area.