Emergency: Trying to change oil pump on 65 again

Ok before I mess up the head on the oil pressure relief valve what should I use to remove it? This actually appears to be an original oil pump with the Chrysler #s still on it. Everything looks good. The Melling rotor and seal kit is no longer available and the pumps are expensive. This one looks in working order so I'm just going to reseal it. I've located the oil pump to block gasket, the oil pump cover o-ring and the oil pressure relief valve gasket. I just hope the spring is fine. But I need to get off the bolt first. It's got a totally square head. What do I need to use to get it off?