I don't know about down there Rick but brass screws are getting hard to come by up here. UHMW is bad for creep meaning that it will continue distorting with pressure on it forever, literally. You didn't say what kind of screw but you may need to countersink the holes and use round heads. I tried the same runners on mine but my sled was narrower and I tried extending the runners out past the ends for more control but the UHMW was too flimsy for that so I took them off and went with hard wood ones.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.