i have read up on this, and i found the best bang for the buck is a fedco 2-342 heater core. i just bought one for 30 bucks at advanced auto, but it was dented, so, later today, my exchange should be in.
i just started water cooling a matter of minutes ago, and i am using a huge aluminum pot for a res, so i can test for leaks and so on, without my core.
like you, i plan to do an external water cooling rig....well, i plan to use a second case to hold the water goods, and i run the hoses to my pc case.
i picked up on a maze2 waterblock here at the forums a few days ago, and would like to know what waterblock you are using.
as for the heater core, i think the one i have is for a 1990 caprice, although, when i look for a core for the caprice, i get a slightly different part number.
this thread shows a chart which suggests the 2-342 heater core has the least water flow resistance, which means easier flow, and less restriction. and, it is plenty big enough to mount 2 120mm fans comfortably:
http://www.ocforums.com/showthread.php?s=&threadid=264046&highlight=fedco