Hello, I am Steve, I have a 1995 Explorer and a 2000 Ranger both 4x4 both 4.0 both XLT's. My grandparent's worked at ford in Dearborn Mich. so I have fords in my blood. My question is can I replace just 1 head the # 3 exhaust valve is bad. Two mechanics tell me I should replace the bottom end...