A high hp/liter engine will likely require 2 keel coolers. A John Deere 6081 (8.1 liter) 375hp only requires one keel cooler. I spoke with the Deere dealer a few days ago and they mentioned they were going to keel cool the new 500hp 9 liter which would require 2 coolers. The customer wanted 500hp, but also needed the engine keel cooled. For comparision, the 425hp version of the Deere 9 liter requires only 1 cooler. I believe for most modern engines, a company like Fernstrum can tell you what cooler will meet the requirements of a particular engine.
A friend of mine needed 2 coolers for his 500hp C9 Cat. Another friend needed 2 coolers for a 370hp 5.9 liter Iveco. I believe you can keel cool any engine so long as you have enough cooling capacity.
How many hours were on those 3406Es?