I asked for help on this on Dieselstop and didn't get any love. Well, I pissed away well over $100 as a result. Don't you make the same mistake. With that, here's the post I posted over there to help other poor slobs.
Here's the deal, the '03 (and not sure of other years) idler pulley can be an expensive SOB. However, not all '03 7.3 are the same. There has also been a change in the part from Motocraft. The ones I'm discussing are heavy cast steel (some have the stamp steel one and this doesn't apply to you). Not like the pulley itself will ever wear out, but the bearings will. Before you drop well in excess of $100 on a new pulley like I just did, think about this. The bearings (yes, bearings) are servicable. Here are the pulleys. The one of the left is the orig one off my '03. On the right, the much bigger new one. They both work on my '03 7.3 and is located top and near the center of the motor (some have an idler under the tensioner setup and some on the other side too, I guess. If that's you, then this probably doesn't apply to you).
Now, if you buy the newer one (why would you), you can't use your idler pulley bolt unless you change the right C clip for the style on the left. If you don't do this, the flange on the flanged bolt will compress against the tabs on the newer C clip (the tabs with the holes in them for the c clip pliers) and the pulley will not turn. Here are the C clips (in this pic the C clip from the new pulley is on the left):
Now, here's where the rubber meets the road. Again, don't waste your money on a new pulley if and only if you have either of these cast steel pulley types. The bearings for the orig '03 pulley are NTN 6203LUA. The bearings for the newer pulley (the big one) are NTN 6203LU. Not sure if there is even any difference between the two bearings. Very common bearings and approx $10/ea. Again, very common bearings used on Honda dirt bike wheels to Toro lawn mowers.