Yes you would need to change the files for the PC to a xbox version.
If you were wanting to put a car that was made for the pc into the xbox version heres how
1.Get IMG Tool
2.Open Gta3.img
3.Open TXD Workshop
4.Open a .TXD
5.Click either VC XBox for san andreas (xbox) or just San Andres (For PC only)
6.Save
7.Go back to IMG Tool
8.Replace the car you want to put in
9.Save it
10.Upload the gta3.img to the xbo
11.Clear X: Y: Z: and E:\Cache
12.Enjoy!

Wrong. You cannot just copy the gta3.img over to your Xbox OR just somehow simply convert the files inside it. GTA3.img has
of files in it so you could not convert all of the files as it would take you days or even weeks to convert all the files. Although you
convert the car's only if theres not to many. Does convert the car's you want and send them over to your Xbox as long as they aren't very complex car's with large file sizes.