My '78 was orignally a 305 automatic, but I converted it to a 4speed with the bigger flywheel setup. I had a 350 and a 400 motor. I installed the 350 because that was the flywheel I had. Since then I have heard of balance weights you can buy to change a 350 flywheel to 400 balance. Does anyone have any experience with them? If they work ok, I may eventually install the 400.