The mill's angular contact bearings now have over a year and a half of use on them. Upon inspection, the bearing showed unusual wear, especially the bottom one. I'm fairly convinced it's a combination of mild contamination and lack of lubrication (due to the vertical spindle the grease tends to settle out of the bearing). Ultimately, I decided the best solution was sealed AC bearings. While they're readily available from an over-seas vendor, those bearings are ABEC-1 which have 0.0005" runout. SKF makes a 7206-BE-2RZP sealed AC bearing which is ABEC-3 and is supposed to have ABEC-5 runout. They're more expensive, but lower spindle runout is particularly critical in a mill (due to uneven chip loading on mill tooling). The one place I could readily find them (and for a good price!) was www.123bearing.com.
I have kept my preload at approximately 100 to 125 pounds.