And I was telling you to go seek out trainers like the GTAIV Native Trainer and all the others that float around every major GTA Modding website.
Something like that should have a "Granite" "Super Heavy" car option you could use. Alternatively you can modify the car files to make them heavier yourself, you can find instruction on that via the internet.
Standalone Mod for what your asking I have no idea. But while your searching you could give it a shot.