the metal sleeve in the bushing may have dimples inside. if it has the dimples, the bolt rides on those dimples so the trackbar can rotate. IIRC, control arm bushings also have the dimples inside the sleeves. only problem with using this type of sleeve is that dirt/rust/mud can build up inside the sleeve.
you can drill the dimples out, along with the axle bracket holes, and use a larger bolt. if you have a welder, you can make your own flag nut.
if the bushing sleeves dont have the dimples, you want to use a bolt that fits the ID of the sleeve while still allowing the bushing to rotate in the mount, there should not be any slop. if the bolt is too small and the bushing is loose in the mount, you dont want to have to tighten the bolt so much that the bushing binds in the mount.