1968 318 to 1973 340 water pump interchange

did you use the torque converter/flywheel out of the 73 340 car, or an after market flex plate w counter weights? 68 318 is internally balanced and 73 340 is externally balanced. If not, you could run into vibration issues. Unless the engine has been altered and is now internally balanced and then you wont have any issues. I ran into this, and the vibration drove me crazy. Installed a TCI weighted flex plate and vibration went away. I ended up leaving the correct 73 water pump and timing cover and installed a 70 and later power steering pump. I just put a 70 or newer radiator in the car as well. I found that to be a lot less hassle (easier to come by the parts) and the 70 and newer radiators tend to flow and cool a little better because inlet and outlet are on opposite sides. The timing marks being off would bother me, although you could install a timing tape, but im anal like that. JMO