Blaine,
How long ago on the new filt, I still suspect it as you say fuel press is dropping off to 1 psi that is too low, mine drops to no less than 2 but comes back up to 3-4 at that speed once I stop the acceleration demand and hit cruise speed, If you picked up some bad fuel and you aren't running a pre-filt you could be partially blocked when it happened to me I couldn't understand it either, my filt was only 5K old so I thought it to be new also, truck ran like a raped ape empty on way to trailer dealers.
When I hooked up my new 5600lb, 9T rated GN trailer, the truck fell flat on its face; at 45 mph truck started bucking and stalling, I could only do 25 up a hill, running without the trailer all was back to normal. I called RATDOC for his .02 he said right away bad filt, I had my doubts but changed it anyway, sure enough that fixed it, next pull of that trailer it was loaded with a tractor and implements. 18,700 GVR tow up to 75mph never below 50 on long grade. Change filt and see what gauges say then, if no difference save filt as a used but good spare.
If you still suspect the cat, disconnect the cat inlet flange separate the joint about 1/2" or so, and see if it helps letting the flange leak. won't be too noisy; the turbo knocks down most of the loud noise.