Any ideas on removing a fill plug?

[1969k10stepside]

Well,now that the motor is in the truck I'm going over everything tightening down bolts that came loose or were never tightened.I'm also topping off the diffs with fluid..I got the front plug out pretty easy with the crescent wrench but the back dont wanna come off.Why can't they just use a regular bolt?They have to use those damn square plugs....Any ideas?I tried the big crescent wrench and it's too big but the little one doesnt seem to do the job.I found a metric wrench that fit on there and I think I bent the wrench before the plug budged...It's the fill plug on the side of the diff case near the pinion on the 12 bolt rear...