RTV won't stick to a wet surface. The non-hardening Permatex (stinky, in a small tube)is good and really all old timers used back in the day. Gasket shellac isn't meant for that use.
Teflon tape or paste will work. Hylomar blue works. FelPro TIGHT is both a sealer and locker.

Or you could put a little stop-leak in the radiator and it'll stop leaking, without disassembly.

R.

PS: I vote for Stop-leak. BTW, that isn't the worst thing you can do from the front of the engine. It is possible to use a bolt long enough that it penetrates the cylinder wall.