It really should not move at all. I replaced the left case on my 99 just a few weeks ago. After breaking it in and running it hard for a few hours I have zero play whatsoever. Replacing a case half isn't a problem. I would be more worried about the problem you had installing the main bearing. They don't like to be side-loaded. I don't think it will be a major problem, as I was riding on my worn bearings that had both vertical and horizontal play. I would ride the bike and watch the movement very closely to see if it gets worse. Also, heat the case in a toaster oven or with a propane torch until water sizzles off of it and then a pre-frozen main bearing will drop right in. On a related note, if you need a new main bearing, I have an extra one that I don't need. Either way, good luck.