I'm running a SBC 350 in my '77 40. The engine heats up pretty good at highway speeds, but always hold steady at its normal operating temp when idling or city-type driving no matter what the ambient temperature is. I'm not leaking coolant anywhere, and the radiator is always full when I check it cold, but the reservoir pretty much empties out every few days, probably a quart or two of coolant. My suspicions are that I am burning the coolant. No oil in coolant, no froth in the oil. On startup, I get a white puff of smoke for a second or two, and then it goes away. I'm guessing either there's a slight crack in the head or block, or the head gasket is slowly going. The engine runs well and I'm not looking to tear into just yet. I saw at the local parts store they have some type of leak repairer (not the Barr's leak stuff) that requires you to drain the entire system and run the stuff through with a few quarts of hot tap water for about 20-30 mins, then drain completely and refill with coolant. Has anyone used this stuff with positive results? Any suggestions/comments are appreciated.