Sounds like you have the wrong thermostat and gasket.

Buy a genuine Motorcraft thermostat and gasket. RockAuto Auto Parts sells them. The aftermarket ones make a lot of assumptions and design changes that are not called for. They are substituting an universal application thermostat and trying to make it work in a wider array of applications instead of actually making a thermostat specifically for your vehicle. The genuine Motorcraft thermostat will fit better, last longer, and will function properly in your vehicle.