I think you have been given very good advice.
The 460 is a capable motor, and Marty nailed the GCWR at 16,000 pounds. That means you can't quite tow 10,000 pounds and stay within spec, only because your van's unloaded weight is surely over 6,000 pounds.
That said... while being the quick up a hill is probably not realistic, if should be able to make it no problem without overheating or eating transmissions. I think the fan clutch is a good place to start. If your van doesn't sound like a big-rig when pulling hills, your fan-clutch is shot. It's LOUD!
Also, high trans temps can also indicate your Torque Converter is not locking up. When you slowly tip into the throttle do your RPMs go up as well? Or to they stay steady as the engine makes more power? If they are rising with throttle position, it is not locking up. The most common cause for this is not a transmission problem however, but a faulty Coolant Temp Sender (it uses a different one for transmission operating than the one for the dash). Faulty coolant temp sender makes the transmission stay unlocked, because this is part of the warm-up strategy.
Finally, as mentioned above, debris from the old transmission is the most common way to trash a new transmission. If you don't replace BOTH coolers, then Ford requires installing a MagneFine in-line transition oil filter. I have one on my van, and don't even drop the pan anymore for service. Just swap MagneFine filters and some fluid.