If you do not know, LOD stands for Level Of Detail.

It is used so that a visual will not a big Frame-Rate killer when you view the model from very far away.

But if this is not the case for you, then I can still help you.

First split your exterior model [in one scene in gmax] to make a clone of the exterior. Delete some object and polys of the new clone so that it can be used as an interior model. Group the new clone with the tag Interior and group the original model as Exterior.

Make sure that both groups are within the same scene. If not, merge them together by going to File > Merge > and the select the scene to merge with.

Save your project.

Select both groups [Exterior and Interior] and go to File > Export Selected. Export as a *.mdl file and start exporting.

After the export process, test out your new model in FS.

NOTE: One thing you have to be aware of. Make sure that you don't have a set of grouped objects within any other group. Otherwise, you might see some serious size problems in FS. This is problem I encountered when I made the Mercedes and making the Porsche. It's a good thing I solved it.

If you can't export as a *.mdl file then export as a *.bgl file by selecting the 2 groups and click on Export Selected and export as a *.bgl file. When the Makemdl.exe program comes up, go to Windows Explorer and go to the location of where you are exporting. Look for 2 files. One of the files should look like <thefilesname>_interior. Rename that file so that it has this suffix at the end <thefilesname>_interior.x

The file should then look like a Text file.

Then go back to the Makemdl.exe program and go to the LOD tab. Click Add and select the new file that you renamed. That same file should appear in the list in the LOD tab.

Go back to the first tab and select Aircraft as your model type. And then start exporting.

This should do it.