Does it matter what side the heads go on engine of 1991 Ford Mustang?
Yes, it absolutely matters which side the cylinder heads go on a 1991 Ford Mustang engine. Cylinder heads are not interchangeable side-to-side. They are specifically designed for their respective banks of cylinders and are not symmetrical. Putting them on the wrong side will prevent the engine from running correctly, if at all. Features like intake and exhaust ports, coolant passages, and potentially even bolt patterns will be incompatible.