I'm guessing you tried torqueing the nut a bit with the pin still in place? Take the inspection cover off the back of the pump as well, gives you good access, and you can look through there with a flashlight in the plate/AFC hole.
I usually go 180 ft-lbs when I torque the nut. Never had a pump slip. I put my new pump on Tuesday at a buddies house, he only had a wrench that went to 150 ft-lbs. Torqued it that, first WOT blast she slipped and killed the truck. Spent two hours on the side of the road resetting it again. Wouldn't have been bad, but had to make two trips back in to town (was about 10km outside on the highway) to get pliers and brake clean. Lesson learned, anyway.