I was just answering this exact question to someone else in a PM, so I'll just re-paste that....
The 3D Studio Max plugin suite by Kam - DFF/IO, requires a collision file (.col) on export - you can use the ones from Vice City's vehicles.dat using Steve-M's coleditor... I have linked all these tools in drugcrew.com's links area:
http://drugcrew.com/index.php?pg=links under the heading "Modding Tools". Make sure you follow his install instructions like a handrail - if you get the installation perfect, they integrate really well into the sidebar in 3DSMAX.
The best method to follow is importing an existing vehicle, then exporting without any changes, making sure to select an appropriate .col file and point to it in the exporter. TXD Workshop is also handy to do the textures. Use of the eye dropper tool in the material editor is also a wise step when importing models, so you can apply GTA materials to them. Pay particular attention when importing, to the model hierarchy - its the button in the toolbar like a flow chart... you need to parent the imported meshes to the part you're replacing's place in the hierarchy.. feel free to ask me if you get stuck, but do give it a try first - you'll get a lot more satisfaction if u nut it out yaself.
Good luck!