these are not silent non vibrating motors.
are the motor mounts good? and your sure its not a misfire?
Well, I just came in from the garage where I have been cursing the napa employees for about an hour. Any how, one thing that I would like to contribute to your quest to find that perfect idle is the ASP UDP (underdrive pulley). The harmonic balancer is part the crank I am suspecting? With this on the SOHC as well as the OHV. With that said the motors share quite a bit other than being from Ford and 6 cylinders. I would give it a try, I replaced my motor with a reman. that came out of a 96 ranger. The odometer said 96k on the body so I am unsure when that motor was taken apart though with the stamp on the block and on one of the heads. I can imagine that there would be less mileage than 96, in short to put sense to the ramble. My idle has greatly improved with this addition as well as a blackmagic 180 xtreme fan. Even with a belt that was an inch long the idle was noticeably calm and I felt the difference instantly after taking the truck for a spin around the hood. Good Luck. Oh and run some seafoam gastank, crank oil, brake booster, the whole 9 yards.