66 mustang overheating??

First things first
You do not have a V6 engine. Look at your engine, see how all the spark plug wires are in one line? You have an inline or I6 engine.

Overheating is a common problem and a search of this site will provide a wealth of information.

However, to get you started:
A thermostat stuck open would not cause your problem as water could flow so you would have a problem with the engine not coming up to temperature fast enough. However, most thermostats will break in the closed position, blocking water flow to the engine. My first step would be to replace the thermostat, cheap part and easy to replace. Just be careful to put it in with the correct orientation, the flow direction should be indicated on your new thermostat.
One function of the radiator cap is to allow the engine to build up pressure which increases the boiling point of the antifreeze/water. A bad radiator cap allows release of pressure and you sould see steam from the radiator cap. This does not sound like your problem, but replacing the cap as a preventive maintenance measure is a reasonable idea.
What do you mean the radiator is cracked? If you add water does it flow out these cracks???? Seems like you would have mentioned this???
Since these cars did not have overflow tanks, the proper procedure is to fill the radiator to about 1" below the neck, just cover the tubes. If you fill the radiator to the top the excess will flow out the tube when the engine heats up. This is normal for an overfilled radiator.
The smoke you mentioned is a very bad thing, you could have already caused serious damage to the engine.

micjoe said:

It will smoke heavily after 10-20 minutes of driving.

Any help ?

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Do you mean the engine smokes, or the radiator spews steam out somewhere. Check your lower hose and see if it is really soft to squeeze together. If so, toss it and get one with a spring in it, the old one is probably collapsing in on itself after the engine starts to pull the coolant through it.

micjoe said:

No the engine does not smoke. it smokes (steams) from the raditor and when i slowly take cap off, smoke (steams) comes from the radiator...

It only leaks after i have drove the vehicle....i guess i will look at finding out how to replace the theromstat its to my knowledge that would cost 8 bucks and hosing is like 22 bucks.

i will look for a manually hopefully and replace it..

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You don't need to replace the housing, just the t-stat. Did you check the bottom hose? It could be the problem. As far as changing the t-stat, just put the big round part (sensor) down (inside the block), and the bell looking part (top) should be exposed until you remout the housing. Buy a Failsafe t-stat, so even if it does go bad, it remains open. It's 2 bolts, just make sure you get a new gasket for the house, and it's a matter of tightening it back down. Don't go all muscle man on it, you will crack the housing, and then you WILL need to get a new one.

micjoe said:

Where is the thermostat located on the engine block? near raditor?

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Have we established if this is a V8 or a 6 banger? Anyways, these instructions are generic enough to work for either:

The thermostat is located on the front of the motor, under a metal housing. Basically follow your upper radiator hose back until you see where it is clamped on the front of the engine. The thermostat is located inside that metal housing that the hose is clamped to. You must remove the hose and the 2 bolts that hold the housing to the motor in order to replace the thermostat.

You will also need a new gasket--they are often sold separately and not included with the thermostat.