Well, the pre 99's had mechanical speedometers, whereas 99+ is electronic. Autometer's cluster is no different, so that would probably be the main problem.
Hopefully someone can chime in with more reading material for you.
Just a thought. I'll bet if you ordered something you would hear from them asap. Good customer service is a lost art. How about calling the dealer that you might order from and let them answer? Just another thought.I do like my Autometer stuff.