Let it Snow, Let it Snow, Let it Snow...

[[SAE]Grim]

As long as the car changes in form, length or height w/e the handlings will change slightly : )
This aint because of the mod itself of course but because of physics in san andreas
Im not sure if the weight of the car changes or not but
If the weight change you will get handling differences because of that.
If the weight doesnt change, you will get handling differences because the original weight  doesnt correspond to the length and height of the modded car

Another factor is that the placement of the wheels will change when u install a mod and therefore the handling will change a bit there aswell.

But its only slightly. You wont get a totally new racing car by installing a car mod

But I doubt you will notice this on a large truck anyway ^^