i thought the power band of the 460 was around 2400 rpm. when i pull my toyhauler up the conejo grade coming back home, thats a pretty steep hill, i keep the rpms around 2400 drop her in 2 and move on up the hill around 35 mph. i'll get there just a little bit slower.
1996-f250-460-E4OD-4x4-crew cab

 

I have a 1996 F350 4x4 Crewcab E4OD Automatic. I pulled a 26 ft travel trailer trailer for 10 yrs and then a 33 ft 5th wheel for 3 yrs. I had 2 long grades like you described everytime I took a trip up North. I noticed the exhaust back pressure that several guys have noted, and I had my exhaust replaced with larger tubing. Funny enough my Cat and Muffler already had large tubes but they were necked down to smaller pipe. This did make a difference and reduced the back pressure noises..... But the issue you are stating, and you are specific about a tapping sound after a long, high-rpm pull, sound like oil starvation to me. The oil is not returning to the pan fast enough to keep the internal ports thru the lifters etc. pressurized. The lifters get hot and sticky which leads to a tap tap tap until everything cools down and the metal shrinks back just a mm or so. I have heard of others having that issue, and I had that issue on an older Ford 390. Lots of work to clean out the oil return ports from the top of the head down, but worth it... Your machine is barely broken in at 95k miles or so... Good luck

 

Friend's 460 used to do it too after long high RPM runs. Changed the oil, added about a quart of Marvel's Mystery Oil. Ran it for a couple hundred miles. Dumped the oil out, filled it back up with good quality diesel oil and a nice WIX filter. Hasn't had an issue since.