I have a lot (over 60K miles towing at a GCW of 17K) of experience with the same drive train as you have, but I have the 4.10 and think you might have the 3.73 and something doesn't sound right. Now I have a BTS "BUILT" tranny, but I have had the bypass go bad and that drove the tranny temps thru the roof and I think you would have problems temp wise even not towing with a bad bypass, but checking that is very simple and takes less than 10 min. Mine went south within 50K miles, but some go forever it's just the luck of the draw. Also, the fix is about $50 for a new bypass and maybe 30min to install. As mentioned you should have the in radiator cooler in addition to the external OTA cooler, but the early Super Duties did not and that is something you should confirm first and again is less than a 2 min job.
I know you have sunk a lot of $$ into this so far, but I'm a firm believer in bullet proofing that tranny and taking that only potential weak link in an otherwise legendary power train out of the equation. If it was me I would beg, borrow or steal and pony up the $5 to 6K and make a trip to Lead Hill, Ar and have Brian do his magic and you will never have to worry about tranny issues again. With your location that is should only be about a days drive hopefully. I have my tranny done back in 2004 with the factory warranty on the tranny still in effect with only 34K miles on it and have never looked back.
Larry
