so a sleeve is what you're after for what is basically a floating bearing seatup when you step up to a better bearing. the original is flanged to prevent it from going into the hub more than it should. Sounds like it's a difficult match for 5/8ths and the bearing inner. Basically a reflection of engineering by accountants, plain tubing for a hub with a wheelbarrow bearing vs one with a tube machined slightly oversize to allow the bearing to stay in position and a proper bearing. id get a 5/8ths ID tube with 1/8th wall thickness then find a good sealed bearing to match id, od and the inner race and sleeve OD. of course, by the time you go through all that, it may just be as cost effective to switch to a different hub.
Here's a flanged, non-protruding center bearing with correct od and id for your hubs/axles
http://spbusa.thomasnet.com/item/inc...=prod&filter=0