I found a pretty good list of heater cores.
http://www.overclockers.com/tips1031/
2-651 looks pretty good. Could put a pair of 120mm side by side.
2-696 is even better 8x5 inches.
Semi-related info:
I bought this on ebay. Looks nice, hope it works
http://cgi.ebay.com/ebaymotors/ws/eB...ME:B:EOAB:US:6