I think there are two possible problems. The ML55 has the spare tire in the trunk, so you'd have to relocate that...
The other problem, if you want to call it a problem, is that the ML received an upgraded interior in 2000, so the '99 seats will not be as high quality.
Other than that, I'm sure that there is a way, it'll probably just be a pain.

Hope I helped a little.

Matt