If he gets a pump that will actually fit, he won't have a whole lot of pump pressure to work with. If he wants to maximize radiator efficiency, he would do well to minimize overall pressure drop.
I know some restrictive blocks like WW work very well even with low flow, but you also have to consider how well radiators work at low flow.
As you can see with this chart,
thermal resistance starts to go up dramatically with flow under 2 LPM. And this is for a fairly large radiator, I bet the effect would be even more pronounced with smaller rads.
edit: img tags
edit2: I suppose the chart better shows that air flow has more of an influence on rad performance than liquid flow. But if you are forced to use a very weak pump and the flow turns out to be less than 1 LPM, you can see where problems with rad efficiency start to arise.