1. Get hold of Img tool 1.3, search google if thy have no knowledge where to get hold of this program
2. Open up gta3.img with the img tool
3. export a backup files ( carname.dff & carname.txd ) of the car which you're going to replace ( in this case Stallion )
4. Delete these files from the gta3.img
5. Import the new car files to gta3.img using img tool
6. Add handling line to handling.cfg ( replace the line which car you replaced )
7. Add colors the same way to carcol.dat
8. Add the default.ide line from setups.txt to default.ide, to it's designated line
9. Get hold of dmagics' wheel mod, check crappy-mods forumz for the URL

Play the game and be happy ^-^, greets Yazu