A couple thoughts:

First, you're talking about a 14 or 15 year old vehicle.  The tranny has a
right to need attention by now.  :-)  If you do have tranny trouble,
remember that you usually have no idea what the previous owner may have done
with a used vehicle.  Check the condition of the transmission fluid closely.
If it's dark or smells burned, beware.

Second, any vehicle that is used for towing needs to have additional
precautions taken on the transmission.  First, you must have adequate tranny
fluid cooling.  Even if it has a factory cooler, I'd add a larger
aftermarket one.  Heat is the enemy of an automatic transmission.

Last, I really like the C-6.  Until replaced in recent years, it was always
Ford's most durable heavy duty automatic.  It's only a 3 speed, so you don't
have all those growing pains experienced with the early overdrive
transmissions.  I'm pretty sure it's not computer controlled, which is
better for that era.  The best part is that they're much cheaper to rebuild
or replace than any of the other more complicated heavy duty automatics.

CJB

That would be 4WD, not FWD. FWD means Front Wheel Drive, and there is no
such thing in the F250.

Expect the 351 to deliver in the range of 12mpg.

As for the transmission, I was told that the C6 is the good one. It is all
manual operation, and is both reliable and repairable. The later models are
electronically controlled, and these have problems that are difficult to
fix. I had the C6 in my motorhome, and it worked great. The downside of the
C6 is that it hasn't got overdrive. It is strictly a 3-speed unit. The
electronic units have overdrive, which (in theory) should give better fuel
mileage.

You may want to check the trasns model.  If it is a 3 speed,
it is probably a C6 which I do not recall as being available
behind the 5.8L in the '92 year model.  If it is a 4 speed
auto, it will be an E4OD.  These transmissions did have
problems but, are still a good choice for the vehicle.  If
the trans was built in later years, it should have had at
least some updates done since many of the original parts
were not available  after a couple of years.  Properly built
and updated, the E4OD is a good trans.  If you, in fact,
have a C6 you have one of the best gearboxes Ford ever built
with pretty rough fuel mileage these days.  In any case, if
either of these transmissions are in good condition, there
will be no problem towing a 16" fishing boat.  Hell, I could
do that with my Crown Vic if it weren't for the beach part
of it.

Lugnut

So you'll know, if you have C-6, it's strictly a 3 speed.  If it has an OD
cancel button anywhere, most likely on the column shifter, it's a 4 speed.
On an F-250, the 4 speed would almost have to be an E4OD.  If it were an
F-150, you could have a 4R70W

CJB