Thanks, guys. I pulled the air pump off yesterday and through a LOT of trial and error and trips to the parts store I found that a 6PK922 (2345mm) belt works best. I had read several threads here stating that a 2257 mm belt was the right replacement, but I can definitely say that on a 93 300e 2.8, if you remove the airpump but keep the same belt routing it takes a 92 to 92.5 inch belt.

So far so good, but I've gone a day or two without squeals before right after tightening the tensioner. I'll probably replace the pump bearing and put it all back on if this proves to be the long term fix. I don't think I'm going to notice any mileage or performance gains either way, so might as well do the right thing.

However, on a related topic here, I removed the tensioner assembly yet again to make sure that the tensioning element wasn't "spun", and it's fine, but what a lame design! Between rotating freely all the way clockwise and hitting the underside of the water pump stuff, and rotating all the way counterclockwise and hitting the power steering bracket or something, there is only about 1/2" of travel. Is this right? How are you supposed to tension much of anything with that?