You could make life easier for yourself by using the Blender physics option fh dist that enables a 'cushion' around/ underneath all models that have it enabled. This would allow floating without the need to relocate a models -x/y co-ordinate. And since it is a setting rather than being fixed it allows the feature to be changed mid game (gravity mine? gravity push? etc etc).

I know people seem to be reluctant to use Blender but it is GPL software and it does all you need. It would also mean eliminating this dependence on old software like GMax which is obsolete and difficult to export from. It also means you will learn a modern 3d program, as the 3DSM interface has changed a great deal (it (GMAX) is already 8 versions old). Lastly with GLSL, what you create in Blender render wise is what you will see in game (90% with a good PC), and Blender is multiplatform, ie Linux, Mac and Windows.

Rant over.