I sure wouldn't assume to speak for Mark since he's the expert here, but I would guess that one reason he is recommending the 6.0 is that if you don't have the in radiator oil to oil cooler only the air to oil one then the 6.0 has sufficient capacity to hopefully do the job of both or at least better than other aftermarket air to oil coolers out there. If you have an existing oil to oil cooler then the lesser coolers like the Tru-Cool 40K etc. would be O.K., but w/o that oil to oil they are not the best choice for a single cooling system addition. IMO you also have to be a little careful about stacking these oil to air coolers in line or even in parallel since there is a fine balance between flow rates, back pressures and inherent cooling capacity. Even the experts got it wrong at first with the initial cooling system design for the 7.3L/4R100 combo in the first iteration of the Ford SD trucks.
Larry