Chances are that your steering gears are worn. The gear unit can be rebuilt and since you have a Mustang later than 66 steering gear box can be removed and rebuilt or exchanged for a rebuilt. Check around. Also check your tie rods and other steering associate components. If when you are driving the car and it tends to drift and you constantly have to correct then you have wear somewhere. Steering gear boxes are expensive and cores are needed to get the price down so be sure that your problem is in the gear box. Don't you work in an autoparts store????????