[HO455]

Did you jack the front end up so the radiator water level was higher than the back of the block when you were burping the air? You have to make it easy for the air to get out. What was the ambiance temperature outside when it overheated? What size of radiator is it? Have you compared the radiator inlet temperature and the outlet temperatures? After sitting so long have you checked the air path through the radiator? It wouldn't be the first radiator plugged up by mud from insects. Just some things to check before you start tearing things down. Most likely the water pump is good. If you not convinced though you could pull it and check the clearance on the impeller and the overall condition of the impeller.
Good luck with that nice 503 green truck.
If you can post a photo or two of the engine may be someone with sharp eyes may spot something.