Most likely causes have been addressed already, I just want to add that locking into a lower gear and crusing the hill at a higher rpm like you did is the BEST thing you can do in that situation. The higher rpm circulates coolant faster, runs the fan faster, and keeps the engine in the fat part of the powerband. Locking in 3rd keeps the trans from hunting and has a better chance of keeping the torque converter locked. The modular Ford engines are all overhead cam and designed to run.
I have pulled numerous long grades here in CA including Towne pass East in Death Valley from Stovepipe Wells at sea level to the summit at 4955ft in around 16.5miles. Average grade is 5.6 percent with a max around 9 percent, but once you start is one loooooong pull! Once the Expy downshifts I manually select the gear to hold it as long as necessary, or I will sometimes manually downshift before the computer wants to. If I downshift I always lift off the throttle for the shift.
