JB weld is an epoxy. Epoxy actually stands for an entire class of glue. Epoxy explains the chemical reaction that goes on to harden the glue. I'm guessing that "Epoxy" brand is a generic brand.
You start with two substances, mix them together, and then the reaction starts. You have a few minutes maximum to apply the glue before it is too hard to work with.
If you get JB weld make sure to keep track of what lid is what. If you put Lid A on jar B and vice versa you get 2 lids that are glued on solid and impossible to remove. I know because I've done it before...
JB weld is good stuff. It comes in rather large quantities though. Probably 10X more than you need. A small tube of generic epoxy will probably do the job and be a lot cheaper. Just be sure to mix it before application if it comes in a tube like this.
You can't just squirt it directly onto the surface. It works better if you use a wooden stick to mix it up on a piece of scratch paper or cardboard first. Then use the stick to apply it. Just remember to work fast. You only have a few minutes. If you want more time you can cool the glue down. The reaction partially depends on heat, so if you cool the glue down it will harden slower and you will have more time to work with it.
I recommend getting a scrap of cardboard. Squirt out a generous quantity of glue (more than you think you will need). Mix it up well with some sort of stick by stirring it on the surface of the cardboard. Then use that stick to scoop it and spread it on the surface you wish to glue.
And yes I think it is plenty strong to hold a radiator. Fun fact, the Lotus Elise and Lotus Exige sports cars are epoxy glued together instead of welded to save weight. They are also aluminum.
Have fun glueing! Fair warning though. Epoxy stinks. Nothing terrible though, but it isn't great smelling. It goes away once the glue is set.