The 300I6 and mazda 5spd is not such a bad combo as everyone lets out to be. I do alot of hunting and fishing, summer and winter. My regular haul is an average of 1 to 1 1/2 hrs. I have a 94 f150 4x4 I6 mazda 5spd, 3.08 gears and a lockright in the rear. No major mods, K&N filter modified airbox, electric fan, everything else is stock. Winter hualing is with a sled in the box and 2 on a trailer @2500lbs and tows it with ease in 4th at 90kph. My summer hauling is with a quad in the box, 14ft boat on topper, 20hp outboard, 28ft travel trailer, and other gear, on the scales 6130lbs. Tows fine in 4th but you are shifting alot back and forth from 3rd on the bigger hills.
I have had it since new and 60% of its 114000kms is towing. I have done 1 oil change on the tranny and that is it, changed to synthic oil. The only problem I had with it was the slave clutch cyl. kept leaking due to the cold temps in the winter-- high pressure on cold plastic parts don't mix.

Ford rates there auto tranny higher on the towing capacities but doesn't include the tranny cooler. If I were to swap out I would go with the NP manual. you can rebuild them with other NP series trannies to get some insane low end gearing, Ideal if you don't want to do a gear swap(rear end)