[Gooserider]

BillA's research and testing indicates that the order of components in a loop doesn't make any measurable difference in cooling effectiveness or CPU temps. It is more important to arrange your plumbing in a way that will give you the best flow path (minimize bends and hose length)

If the heater core you are looking for is for large trucks, you might try either a radiator shop or a truck repair place. Truck shops have different part supply chains than standard auto parts stores. Don't be to supprised to get a severe sticker shock however, as truck parts tend to be expensive.

I suspect that your cost will be less with a single large radiator than with multiple heater cores. (especially if you go for 2nd hand) It will also give you the potential advantage of needing just a few large fans instead of a bunch of little ones.

I tend to agree with the idea of splitting the flow, as pushing an excessively fast flow can actually damage your plumbing. You also have a diminishing returns point where your cooling won't improve significantly once the flow gets past 2-3 GPM. Given the stated volume of your pump, you should have no problem getting that level of flow through at least two or three branches. If you were to find any one branch getting excessive flow, it wouldn't be hard to put some sort of restriction in that branch to balance things out.

Gooserider