Hello all,
Well, after having owned the 3J for over 20 years, I now have a leak somewhere in the core or the upper and or lower tank. After removing  the Rad. from the tractor, I am assuming those screws nuts etc on the side of the housing hold the core in place correct?  Hope those 6 4B5825 pads are still in useable shape. That part is not available anywhere I checked. 🤔🤔🤔🤔

Yes the cap screws hold it all in place. A good penetrating oil for few days, and a impact on a low setting. More oil and keep increasing the setting on the impact wrench. Some moderate tapping with a hammer also loosens the rust bond. Now you can use heat or just rattle them until they break. Even if they break down in the cast, lay a flat washer and build up the broken piece with arc weld. Arc rod doesn't bond well with cast so it dose not stick. Clean weld and now weld a nut on the flat washer. Let it cool till you can hold your hand on it, but longer is better. Tap on it oil it and try impact again. Try turning it tighter if it has not moved, with more oil. If it moves, work it back and forth. If it breaks weld it again. I have never had one I could not get with welding to heat as well as give a place to turn it. The most was 7 times on bolt. BUT IT CAME OUT with the treads in good shape yet.

Thanks for the advice Ray. Removing the Radiator was no problem. The flathead screws were not that rusted… only surface rust. 👍👍

On the really rusted stuff we get in my area; It's always a treat to have all of the original CAT bolts still in. I've taken some really badly rusted bolts that had little or no hex left out very carefully not heating too much that would melt the solder on the core with my Acetylene torch with small brazing tip. If someone used some run of the mill gr5 bolts, have fun.
Recently, I have acquired one of those induction coil bolt heaters. That thing is nice too.