The special WP tool with the 1/2" drive and short handle is really intended to just turn a freely-moving pump to adjust the belt tension. Although it might work to break a frozen pump free, I'd be a bit concerned about that 1/2" drive linkage giving way if the forces aren't well aligned. If you were able to get that big vice grip locked onto the pump, how about slipping a snug-fitting pipe over the handles for lots of extra leverage? Seems like that would be even better than the slip pliars that I mentioned above. Was there lots of rusty coolant or other crud when you drained the cooling system?