The SU's for a Volvo will fit your current manifold. You are being confused by the term "three bolt" which refers to the air filter to carb arrangement. SU's fitted to the 122 model have 2 bolts securing the air filters.. You can use earlier SU carbs from a B18 motor but will need to be sure the needles are meant for the 2 litre. All the Volvo carbs--SU and Stromberg use 4 bolts to attach the carbs to the manifold. Before changing anything, squirt a spray of water vapor around the carbs and manifold while the motor is idling to check for vacuum leaks--a common cause of poor running and high idle.