I feel your pain. When I first got my '91 three months ago (with 180k) it had similar noises to yours. It started out making the light quiet diesel-like noises. I tried lucas, marvel, light oil, heavy oil, etc. Nothing would make the noise go away. The motor had good oil pressure (like yours). After logging less than 1000 miles on the first motor, the #1 rod came through the side of the motor. :explosion
I'm now on round two with a different motor out of a wrecked '93 that had 160k on it. This second motor runs good, but is starting to emit many of the same noises as the first. Obviously, I am concerned. I have little choice but to drive it and take it very easy, because it is my daily driver due to my pickup being down at the moment.
Hopefully this second motor lives long enough for me to get my pickup back together. If the second motor pukes like the first, I'm really not going to be inclined to rolling the dice on any more "used" motors. I plan on tearing into the 4.0 and completely re-doing the bottom end and possibly even going .010" over and installing some new pistons, -and a new cam and all new valvetrain. I did install a new oil pump, rear seal, and timing chain before I put the second motor in, -but it appears that all that may not have been enough.
Like I said, I feel your pain!:banghead: