you figure it out yet?
these trucks sometimes are tight inside that area between the radiator and the engine, the hoses and belts get in the way and make it hard to see the drain valve on the radiator.
If there is not one beside the lower hose or near the transmission line on the reservoir side of the radiator then it may not have been installed, but there should at least be a plug somewhere on that lower tank of the radiator. Some of these cooling systems on these units did not have the usual brass drain valve butterfly type that drains through the center, some of them just had a plastic thumbscrew type drain, when you start to open/unscrew it after a few threads out, it will start draining through a small hole in the threaded area of the plastic thumbscrew so it will start squirting out sideways so be ready for that.
If by some strange chance there is not any drain valve then your only option is to remove the lower hose carefully and use as large a drain pan as possible. Also have some speedy dry or pigmat handy as well because it will make somewhat of a mess if you're not ready for it.
I worked on cat's for years so i would find it unusual for there not to be one on this truck.