I had a similar concern on my 49 216. In troubleshooting I added an overflow tank and haven't had to add fluid since. When cold the radiator is filled to the top. When it gets up to operating temp (180) it'll spit anti freeze into the overflow tank until it gets about 1.5"-2" lower in the radiator resevoir and stay there. It'll suck the fluid back in as it cools.
I've not had any over heating problems either.
I plan on adding a nice stainless or polished overflow tank one of these days but my temporary tank is doing a great job....
Dave
