Gas gauge prob please help :(
 

So around a month ago i bought a 2002 mustang GT with roughly 80K miles with a swaped motor that the man claims have 65K on it and claims that the cluster wasnt swaped either but who knows besides him. Also no paper work for the swao. Before i bought the car during the test drive the gas gauge read 3/4 of a tank. After putzing around the block a few times after i bought it and finally put gas in it the gauge read full which made me happy. But now the gauge doesnt move from full but when it does it goes to right under full line. I am afraid to drive it untill the gas light comes on because i dont know if it will. I dont know the issue and am afraid to have a crack at the cluster since i wouldnt have a clue what im looking at hence im new to cars. I looked up a few videos and cant really get down what i need to do to fix it. Please help me figure out my problem.

Probably has a bad stepper motor. It makes the needle move. There are several repair places that you can send the cluster to that will replace the stepper motor.
I had mine go bad on my 2005.
It's a fairly common problem. If you or a friend are good at soldering you can get stepper motors fairly cheap ($5-$15) and do it yourself.
Do a search for mustang stepper motor repair.

I dont know if it would be the stepper motor because when i do turn the key the gauge does move to full then doesnt move from there unless i turn off the car.

Don't be afraid of the cluster. The speedo needles can be taken off and reinstalled easily. I once had a ford tech put my gas needle on in the wrong position after installing white face gauges. The car was showing empty - needle hanging just below the line when it was near empty. To solve the problem, I simply filled the tank up completely and reapplied the needle right there and correctly. I allowed just a "hair" for the reserve half gallon or so. That fixed it. Also realized how easy my gauge install could have been by doing it myself.

It that doesn't work, then you have an issue with your gas tank float or motor as mentioned. Entirely different headaches.

do your gauge needles all sweep when you push the cluster switch?