Out of interest are you doing the handling & carcols files as well ?
You should replace carcols for the right skins and handling, well, for handling.
Unless the cars you're replacing are the same/similar models to the ones in game, then I recommend using the Realistic Driving EFLC mod which has the best handling around. The guy who made it claims to spend a few hundred hours on it. I can vouch for it being one of the best GTA IV mods.
My suggestion would be to install it first and then replace any particular car's handling once you install it.