The smallest engine you can put into a 1966 Mustang is the I-200 straight six cylinder. In order to put in a 4 banger, you would have to change out the transmission and the rear end in order to accomodate the 4 banger. With the changing out of the motor mounts, bellhousing, transmission and some alterations of the driveshaft and rear, you don't stand a chance of making any progress towards saving money in the cost of gas over the long haul. Stick with the I-6 200