Well, I finished the radiator box, but I'm not happy with it.
Since I suck so bad at soldering (especially with a torch as the heat source), I fell back to plan B - epoxy. While the epoxy was fine for building the actual box, a few problems arose (but not because of the epoxy).
The resulting basic box was not exactly square. In fact, one side is shorter than the opposite side, resulting in a slightly bowed shape to the other two sides. I epoxied the radiator rails to the top/bottom of the box (these allow me to slide the rad in/out without worrying about any alignment issues). This went fine (and served to force the top/bottom of the box into a reasonably flat appearance.
Next, I wanted to install a moutning flange for the pusher fan. I cut it out, bent the opposite ends into attachment flanges, and epoxied one side to the bottom of the rad box. Yesterday afternoon, more than 24 hours after attaching the first side, I epoxied the 2nd side. This morning, I removed the vice grips from the first epoxied side, and the fan mount detached itself from the rad box. I guess it was a sort of bizarre desire to confirm my suspicion, but I prompty re-vice-gripped that side, and undid the vice grips from the 2nd side. Sure enough, the 2nd side detached with a disheartening "tick", and I was left with a fan mount that wasn't attached to anything. It was time for Plan C.
Pop rivets. As much as I hated to do it (pop-rivets are ugly little bastards), I drilled and riveted the rad box and the fan mounting flange together. As if that wasn't bad enough, I think the epoxied radiator alignment rails are coming loose, too.
While it's not perfect, it will do until I can construct an improved version of the box, this time out of aluminum. Aluminum is cheaper, and I won't feel so bad about complete screwing it up, and I also want to change the design of the actual box mounting flange a bit.
I'll post a pic of the finished box tonight.