The 1997 351 had a 200-210 horsepower rating, and torque ratings from 300-328 lbs. ft.  If it has alot of miles the horsepower may be down to 160-180....

How many miles on that 351? What gear ratio, tire size, transmission, etc? More info needed. All that those "cherry bomb" glass packs will do is make it loud, drone in the cab and scare the horses.....  I bet it didn't pull much better than it does now when it was new..

My reply would involve a couple of clarifications: Does your state have emission testing? Are you planning on keeping your truck for a while if you can increase its HP?

If your first answer is no, and your second yes, I would offer that you consider an engine swap for a big block motor. Our previous truck had a small motor, choked by emissions, and was an absolute gas burner. I changed the intake manifold, camshaft, switched to a 4 barrel carb, dual exhaust etc. It helped a lot, but I still ended up with a small block engine, working its tail off.

If you like your truck, and don't want to spend the necessary funds for a new one, an engine swap is a viable alternative. If you don't have emissions, you can go old school with a revamped carbureted big block, that will give you instant gains in HP and torque. If you have emissions, you could consider a newer FI motor from a wrecked Ford vehicle. They also have a lot more HP and torque than your motor.

There are a multitude of accessories that can upgrade the efficiency of your present motor. Unfortunately, most are attuned for additional HP and high RPMs, when your real towing needs are more torque at lower RPMs. You can spend a great deal of money on dual exhaust, air intakes, throttle bodies, programmers and the like. You will realize a modest gain, but little that will give you the low end grunt to tow tons up a hill.

Again, if you want to keep your truck, and are willing to spend a couple thousand dollars, you can effect a power plant upgrade that will tow trailers. It's up to you and your budget, what you want do accomplish.

If you don't like your truck and are on a tight budget, get in contact with HTB, and he can set you up with a good used diesel that will tow your trailer.

Gard

In my opinion you are doing good with small engine and 4.10 gears in a heavy truck.  I would remove the emmis mess and up the exhaust system to at least a 2 1/2 or 3" with some sort of turbo mufflers (glass packs will kill you ears), good quality tune up plugs wires, coil and then a quality aftermarket air intake system so it can breath better.  K&N works well with gas engines.

You maybe able to get close to 15 unloaded driving easy and maybe close to 8 or so loaded but that's going to be it.  If you have the auto overdrive tranny, go buy a good tranny cooler FIRST!