You can pretty much put it over any car you want. Just make sure they have the same number of doors otherwise you can get f-d up.
All you have to do if you want to change it to something other than what it is is just change the .dff and .txd files to the name of the car you want. (make sure you back up all original files)
And when placing the handling and other attributes just copy numbers and not names. hope this helps
Also building up a blueprint and image resource to make cars for GTA IV (hopefully its mod friendly). May concentrate on modern everyday cars to replace the 'b' cars and luxury saloons (Lexus, BMW etc)and the odd car that takes my fancy like the Raybring NSX.