Hi Dude,
I don't know or you've find it, But I've the IMG tools for GTA-VC & SA... .
or something better like Ultimate Editor for San Andreas, and GTA Mod installer(for VC & gta3)They really are GREAT TOOLs!
By Ultimate Editor you can change almost EVERYTHING!!!!
By GTA-mod-Installer you can replace a car with different class with others,! ex. put a Peguet 206WRC instad of BobCat!!
He will do everything Automaticly! what you need is to give your Vehicle a name! and DONE!!!
(oh you've to check that he has took a good Handling for that car!)
These 2 amazing files are larger than 1MB so I can't put them here... but give me some mail, I will send them for you!
Cheers.
gta_SA_IMG_tool.zip
gta_VC_IMGTool1.3.zip