Bid, you can get a higher pressure radiator cap (7# or ?)and get away with it that way, but yes replace the old 4# relief valve and let it run out the overflow tube. These radiators will typically [censored] coolant for a few days after filling the system or radiator while it finds it's level of comfort. That's why the overflow goes to the ground; to let the coolant go. It will often burp coolant when coming off the highway so this is normal until the system "equilabrates" and then it seems to settle down. They all do it this way in my experience.
Be sure to drill a 3/16 hole in the thermostat prior to refilling the engine to prevent any possible air entrapment behind the thermostat (if you drain the system) in the head. This will crack the head and is a COMMON cause of cracked GMC heads in 228-302.
The top tank or the original radiator IS an expansion tank and is designed as such. Adding an expansion tank on the inner fender will handle the overflow but will not properly pressure. Replacement radiators for these trucks do NOT have the original configuration and use an overflow tube off the cap neck designed to work with a pressure cap relief.
I saw your AC number and I have no idea if they can match it from that. It's not an AC Delco part, it's a radiator manufacturer part #.
