Head Unit Size Difference

Hi again guys,
I have a problem to solve here....my Ford Laser Lynx(2001 Model) has a stock cd HU(the kind of big squarish type) Before i want to install my amps, i want to remove the stock HU and swap it for a better one. I bought a new cd HU(half the size of the stock one,the size of a normal HU).but im wondering if it will fit in the same space as the previous HU. Im worried the new HU will be loose or too big in there.( it is an Alpine CD Receiver) i am going to use a pre amp to control the sound signals and volume to the amp instead of the Alpine's electronic pre-amp. After installing the HU and the pre amp, how do i cover up the extra opening on the dash cavity? I don't think the local stereo shop has the neccasary kits that i will need. for your info, im living in Malaysia. and the stereo shops here suck with inexperienced employees. Their workmanship is always shabby. So im pretty worried of letting them work with my car. I learned this by experience. my cousin had his car audio "professionally" installed at a local stereo shop, do you have any idea of how they did their work?! they wired all the door speakers directly to the amp without any crossovers( the amp itself had only a high pass filter). and guess how it sounded? Horrible!! So i need advise from you guys.....any help would be appreciated.

YOu need to buy a mounting kit,  do you know what the equivalent car is in america???  like mustang, taurus???  because there is a mounting kit and a wiring harness that will make the installation easier and look professional