somebody school me, rear sway bar (factory)

[fergi62]

was at a yard early last winter and purchased a rear swaybar from a 75 dually to install on my k-10 p/u. finally got the longer u bolts and tried to install it but the bar itself was actually too narrow to fit the k-10, my question is are the dually's spring seats welded on at a narrower distance than a single rear wheel? i was under the assumption that the rear was the same other than the outside wheel. which i thought was actually taken up by the rear fenders? thanks in advance