SAMI, worst tool ever.
IMGTool is a very basic tool - like SA-MP compared to MTA - in this case, MTA is Alci's IMG Editor, Alci's is a very good program for replace everything you want, adding, removing, replacing, I can say it's like WinRAR opening IMG files - not that level, but it's an awesome program anyways.
Well, I see you're asking for a program where you wanted to make your own GTA mods right? you should try TXD WorkShop for make your own textures and Autodesk 3ds Max with Kams scripts for the DFF (structure) part.
For change the handling part, you must edit the handling.cfg file located at GTA San Andreas Directory/Data (recommended notepad, simplest tool ever ), find the line of the vehicle you want to replace, and replace that line by the readme.txt one.