A weapon modding question.

Wonder if any guys on here can help me figure out what I have to do. Here is my story; I create all my models in Maya(its what I first learned to model with and continue to do so.) Now I import my new mesh into max, My main question is how would I go about changing the original mesh to my new custom one and then exporting it out of max correctly? I've done my own testing and each time all I get is errors, is this kind of weapon modding similar in nature to how Fallout 3/NV works in that you have to have your new mesh with the exact same number of parts as the one you want to replace?

Any help is appreciated.