can I still sell my 1999 mustang with a title loan on it?

You sell the car and use the money to pay off the loan. The buyer gets the title from the loan company.

I don't know of any intelligent buyer who would give you their hard earned cash based upon your promise to get the title later, after you pay it off. A compromise would be to take the buyer with you to the lien holder, and make the transaction there. You really need to check with your state (the lien holder or a bank can also tell you) and see. In some states it is illegal for you to sell the car without the permission of the lien holder, and generally there are some hoops to jump through.

I sold a car with a lien to an out of state buyer, he wrote me 2 separate checks. One to the lien holder and the other to me for the balance. I paid the car off and mailed him the title, he took the car and had trusted me to send him the title and this was for a 91 Corvette ZR1. I also gave him a bill of sale.