I don't know how much strain a packing wrench can take, but they don't look that strong to me. I would think that if a pipe wrench couldn't do it, you'd break a packing wrench. The advantage of the packing wrench is it will fit into tight spaces a pipe wrench won't.
I'd use the penetrating oil (PB Blaster or Kroil, not WD40), and be patient. Takes several applications over several days to work. Then go after it with the pipe wrenches. Biggest problem is its probably in a position where you can't put much leverage on it.
On my Columbia 36 you can't get to the stuffing box unless you're 6'2" and 90 lbs. The marina had a kid who could do it, but at 5'10" and 260, there's no way I could reach it. So I cut an access hatch in the cockpit sole and put in a Lasdrop seal. Hasn't leaked a drop since.