It should be plug and play. Hate to ask the obvious, but you need to line up the tabs on the speedo drive before it drops in? Pictures of your gap might help. Make sure the spoke wheel is XS650 I think the XS360, 400 wheels are narrower.
We have gone around a few fork alignment problems, so again I gotta say this. Remove wheel and fender. Loosen the clamp bolts on the triples one side at a time, rotate the fork tube. The tube should turn in the triples with small effort, does the end of the lower stay in one place? (checking for a bent fork tube) If the tubes are real hard to turn, drop one side down till it's free of the top triple, it should stay in alignment with the hole in the triple and slide right back in. If it has to be forced to line up, the lower triple is bent. It's common after 40 years that someone has pronged the bike.