When I am trying to maintain speed whether it be going up the mountains in Colorado or pulling a heavy trailer, I try and split the difference between the HP peak and the torque peak. If it is a steep grade or heavy load, you should keep the RPMs above the torque peak, but don't exceed the HP peak. Following this procedure I have been able to put 92K of trouble free miles on my '97 SOHC, many while pulling a heavy trailer and many more while going up steep mountain grades. My SOHC likes to rev higher than your 5.0L so I had no problems running for hours on end at 4000 RPM when I pulled a 4000 lb. trailer from Ohio to Arizona. When I got to the mountains in Northern AZ there were a few grades that I had to lock my transmission in 2 (3rd gear) to maintain 45-50 MPH. My RPM was pretty high for the couple of minutes that I did it. On the 5.0L I wouldn't try pulling a heavy load up the mountains in Colorado at 4600-4800 RPM. The HP peak is only 4600 RPM (at least on the '97 5.0L). As you get above that your torque will drop off quite a bit. The HP is what will keep you moving at your present speed, the torque is what gets you up to the speed. I would try and keep it around 4000 RPM or less. You shouldn't hurt anything if you exceed 4000, but I wouldn't want to put the heavy stress on the engine for prolonged periods of time. On the OHV, I wouldn't go over about 3500 RPM for extended periods of time. I remember my '94 4.0L felt pretty weak above 4000 RPM.
