My 43 GMC had a weird habit of spitting out coolant via the radiator cap (down through the over flow tube.). Turns out that the cap needs a gasket to seal properly. $1 item must have cost me 3 gallons of anti freeze.
Other thing I learned is that of the three 40's vehicles I have (42 Ford, 43 GMC, 43 Chevvy) all of them have a different coolant level that they like... and none of them are anywhere near what the book says it should be.
One thing that did help IMMENSELY for the Ford was adding in the felt grill/radiator baffles. At speed most of the air was going AROUND the radiator because the baffles were not installed. So make sure you have all the sheet metal in place and if there is a fan shroud make sure its in good shape.
