From loctite The process of knowing how to remove red threadlocker is a little different than for other threadlockers. The key is to apply localized heat greater than 250°C (550° F). Dan
I'm thinking that is pipe thread going into the block and it was wrenched in real tight 72 years ago so it wouldn't leak. My plan of attack would be to take a scratch awl and scribe a witness mark on the block and fitting so you could see when it moves that first minute amount. Screw in the tube nut. Then it would be Kroil and tap, tap, tap the block and fitting couple of times a day to vibrate them and help the Kroil work its way down the threads. After a few days of this put your wrench in the fitting and tap it back and forth a few times. Check your witness mark to see if it moved. (The reason for the witness mark is because it is very difficult to detect a small movement when using a hammer on a wrench. Hammering on a wrench imparts a shock load to the fitting that will help it break loose with less pressure than a long steady pull on the wrench.) If doesn't move, repeat daily. When it does move, finesse it gently back and forth and work the fitting out. If it does break off, you have a couple of possibilities. You already have a hole that is pretty well centered up, so you can use the grease trick and drill it out to the threads and pick the remaining brass out. The other method would be to grind down a hacksaw blade to fit in the hole and make a couple of relief cuts and hopefully relieve enough pressure to be able to screw the broken piece out or pull the pieces out. An additional thought, grind the hacksaw blade so it cuts on the pull stroke.
I have had to deal with many of such brass plugs buried in steel in the pipeline business . The answers have already been made to the issue. Try heat and a birthday candle 1st , if it doesn’t give up the chase , saw it off and cut a split in the threads . It will loosen the bond nicely and allow removal
Guys & dolls this fitting is in a place on the engine that is not easily assessable & I believe the engine is still in the vehicle.
Put some ice cubes in your freezer and turn it as low as it will go. let them star there for a day take one out and hold it against the brass fitting for a couple on minutes then try ad turn it out using a 6 point wrenchj either box end or socket.
OP: If possible (for easier access ideas) post a photo showing the fitting and rest of engine around it. Hard to discern where it's located in original photo.
Go for it- wrench the hell out of it. If it breaks off, you'll be left with a hole in the middle of the fitting... Stuff a scrap piece from an old cotton sheet down in the hole (to keep fragments from going into the oiling system), then drill it (there, somebody had to mention this option). Size up until you get to the minor thread diameter and pull the threads out with a needle nose pliers... Then use the needle nose pliers to pull out the cotton scrap. I've done this many times and always had great luck. Helps to have a powerful vacuum suction device at hand as well.
Punch it with your fist, let blood flow around the threads then belch on it for a toxic gaseous reaction...Just do something (and then let us know how it went)! The suspense is unbearable.
1oldtimer's idea of 8-point sockets would most certainly work, but I went on the "jungle website" and ordered a Weatherhead set, per Jack Vines post, and it did the trick slicker 'n snot on a barnyard floor! Thanks to everybody!!