So you want to build your own true muscle car? Get a pen and paper ready and jot these things down. A real muscle car has a strong lightweight chassis from an American car manufacturer. While any car model from the late 60's until the mid 70's can be used, it's best if it's a 2-door mid-sized car. Now where can you find a 40-year-old car nowadays? The scrap yards are a good place to start, but finding one in good or even passable condition will be difficult. If you ever find one then it should be stripped down to the frame rails and then reinforced so that it won't twist once you drop a massive engine and it.
Now that you have a chassis for your muscle car you need something to make it go, an engine. This is what puts the "muscle" in these cars. But finding an engine is as difficult as finding a chassis, if not more. The most common chassis that are used are those of the Chrysler cars, namely the Plymouth Satellite or the GTX chassis, because they are the easiest to find.
It's always better if you bring along a reputable mechanic with you whenever you inspect an engine you're planning on buying. He'll make sure that there are no hidden defects that will be a huge problem once you try and mount it and your car. If possible test the engine to see if it will run, and if it does, that there are no weird noises coming out of it. Now if your mechanic gives you the all clear, then purchase your power plant. Recondition it and install any modifications you like.
Now that you have a lot of horses running under the hood, it's time to think about how you can control them. For starters, if you want to go fast you have to be able to stop effectively too. Install a new braking system to make sure that you won't slam into another car while going 180mph; large diameter disc brakes are recommended for most muscle cars nowadays.
You should also consider beefing up the suspension and under chassis so you will be able to gain better control of your car.
These are just the basic components of a muscle car, the possibilities of making your own custom car are endless. You are only limited by your own imagination.