samualt,
It is MY opinion that set screws are in-adequate in the long run by them selves. On my present setup, I have cross-drilled the shaft and used 1/4-20 socket head bolts to hold the pulleys in place.
I am presently up-grading to ball screws and will have key ways cut in all the pulleys and shafts. I don't want them to move
As for the belts, I have been using 3/8 XL belts on my mill/drill and they seem to be satisfactory. I have been using them for over a year now with no apparent loosing of the belts or other problems. I am going to use them again on the ball screw conversion with the only difference being better shielding to keep swarf and coolant off the belts and pulleys, especially the Y axis.
Bubba