My 1957 D47U had coolant leaking around the front head stud that connects the radiator brace. Whenever the engine was running, it leaked quite badly.No signs of coolant in oil or cylinders. I've never had the head off one of these engines.From what I gather coolant passages are sealed with a metal ferrule and an oring.. Then there's a metal head gasket. I'm assuming the rubber oring is bad. The weather isn't very good to be pulling the head. And there's a couple small things I'm needing to get done. So I tried some improper methods to hold out for better weather. Since it appears to only be leaking from around the nut. And everything else is sealed by other means, I tried some rtv. The stud had the radiator brace with a flat washer then a nut. I removed all of that and cleaned the stud. I applied RTV around the stud, forcing a little into the hole around the stud. Then changing the order I put the washer with enough RTV that it squished out slightly. Then I put the bracket, then the nut. After torquing it, I let it sit overnight. Actually due to the weather I think it was a couple days before I started it. I've been running it this way. It hasn't lost a drop of coolant. The temperature gauge stays pretty much 160. 

I have a 7U here that did the exact same thing; not a lot, but enough coolant loss to be concerned. I put a new bolt in, etc., and never had any further issues.
My own thought was how/why is it leaking, but sealing it off has not had any consequences, so all is good! JM

I’ve used the brush apply pipe thread sealer from permatex successfully more than once-

Keep safe-

aviation no.3 form-a-gasket can work wonders too.