I don't think lateral movement will be any issue. If it's that close of tolerance, it won't be notice. Like you, I figure either side if the glass was the biggest thing to take care of. Time will tell.@NikP I'm having this same problem this week and this was the solution i thought i might try. Haven't tried yet. Sounds like it worked for you though?
When i bought my 55 it had no weatherstrip at all on the tailgate. So the glass was rubbing against the metal tailgate and etched stripes up the glass. I bought a new piece of glass that is exactly the same size, but it won't fit in the SLO felt channel. I don't know how much lateral movement the glass would have, but I suspect the felt on either side of the glass is most important to reduce rattle and protect the glass.