A while back I looked into a IC core that could be made in any size I wanted. I was looking to make a double thichness cooler, but there wasn't much interest to make it cheap enough to warrant further investgation. The cores were a extruded "tube and fin design. The fins only stuck out a little to provide more surface area without interducing more turbulance, which makes it more restrictive, plus the fact that there was more area in the core (due to the 1/2"-3/4" that is taken up by welding the two together). Core cost was about $450 just the core for 1, Plus the labor to put the endtanks on (if you just wanted the core that was fine too).
Jeff