Hi. When you say it is a slip fit, do you mean that it should spin freely on the main shaft? It doesn't seem right that mine spins freely. Wouldn't that make it really inaccurate? I've been considering peening in some divots to make it "catch" onto the main shaft. Stop me before I damage a possibly irreplaceable part!
It's a slip fit in order that it can be assembled easily, along with the spacer. There should be no need to peen anything. The above posts should give you all the info you need to make it work. Most likely possibility is the washer getting dished - just flip it over. If the speedo gear still spins with the u-joint bolt torqued properly, then you may have a spacer that's too short (having worn down from the speedo gear spinning). You could put a few center punch marks around the face of the u-joint yoke to upset some metal and make it effectively longer.