Where is your sending unit mounted. My BBC is in the low 20+ range when hot. I have an electric sender mounted above the oil filter very close to the exhaust manifold. I am wondering if that has something to do with mine. What cam did you go with when you rebuilt? Your saying you can idle around 630rpm? I am in the 850 to 900 range with an RV cam. You may be low on your idle speed! I may be high?