If you were closer... I just pulled the rad from my '89. It looked like it was leaking so I changed it. You're welcome to it.

 

I've used JB weld, or even plain old 5 minute epoxy. A couple things. You can add a pinch of normal fiberglass insulation. Not sure where you might find that. Maybe fiber from something else. THEN OPEN UP the hole/crack. You gotta get it open so you can get it in there. Scratch, roughen up the inside and all around. Get it clean and dry then apply the epoxy/fiber. Without the fiber it might just drip out. Just putting it on-top of the hole won't work well. Get it in there, open it up. Then leave the cap loose. You don't always need pressure. Just watch it closely and don't bake it! (and you still have pepper in there)

That actually can work pretty well. I've done "semi permanent" repair that way at least a couple times.