Ben, your heatercore knowledge is truly frightening to behold. Ever consider working in a parts department?
For some wicked rads, go to
www.lytron.com and look under 6000 series copper heat exchangers. They have them in all different sizes and shapes and they will probably kick the shiz out of most anything you'll come across. Be warned, you'll need a good pump for these ... even the small ones will spank the ass of the Via Aquas and Rios people talk about around here. I believe hmale used one of these on his acrylic masterpiece that is floating a month back or so in this forum.