From everything I have read you want something simular to the Fedco 2-342 (GDI 399090, 1972 CHEVROLET TRUCK C10 1/2 TON SUB 2WD, 9.5"x6.125"x2") heater core. You can get it at any auto parts store for about $20 total (U.S.) I just got one yesterday.
The reason you want one like that is:
1. It's a single pass. You will see the in/out on either side and the water doesn't make any U turns and such. This increases flow rate tremendously. A double pass unit can kill head (the ability to pump water uphill) by as much as 70%. Double pass units usually have the two connectors on the same side.
2. The in/out is diagonal to each other and this causes more of the fins to be used. Otherwise the first few fins may be the only ones transfering water because of the low pressure of the pumps we use (as opposed to the much larger pressure in a car cooling system.)
3. It's copper and will match your water block so there is no electrolysis.
Things to change about it:
1. Would be nice to have a larger unit for outside-the-box cooling. A nice 12"x7" size would be great!
2. It would be better if it were 1" to 1.25" thick instead of the 2" thick. This would be for better airflow with the slower fans we use as opposed to the heavy duty blowers on most cars.
3. It would be nice if it already came with .5" ID (Inside Diameter) barbs (the hose connectors) that most of us are using.
You'll have to ask someone else about fin spacing and such.
Just my 2 cents, and I could be wrong
