Do you know anyone who has Final Cut? LiveType has a very cool odometer font that will do this very well and easily. The text characters are on individual odometer wheels so the concept is a bit different than the tumbling letter signs you are emulating.
However, in addition to Ian’s advice that the SFX sell the effect, you will want to use monospaced typefaces and you may need to box each character, as on a real sign. You can easily build the boxes using the Grid generator effect.
bogiesan
This is my standard sigfile so do not take it personally: “For crying out loud, read the freakin’ manual.”