Hi Terry, when you wrote that the fan is working correctly, you didn't mention anything about blown fuses. Is this still an issue or has it been solved?
In any case, Toyota uses shrouds on all their stock designs, including both normally aspirated and turbo diesels. As I mentioned, it's all about the physics of flow and surface area to exchange the heat. Toyota also uses a clutch type mechanical fan with all their current diesel offerings. I would be surprised to see a factory designed cummins powered rig, whether bread truck, UPS truck, etc., without a shroud and a mechanical fan.
I don't know whether the 4BT produces more or less heat than the 2F it is replacing, but clearly the cooling system you have in place isn't adequate, and I would suggest that a shroud would be the first place to concentrate your efforts, to get your existing SCFMs more evenly spread through your radiator than the 40% you wrote about. For a shroud to work properly, you will need the fan placed back somewhat from the rear of the radiator; how much room do you have to work with? An inch or two is what you should be looking for.
Something to consider when people talk about cooling ability is that sometimes they don't take elevation into account. For example, even down in Denver you are losing 20% of cooling ability due to the thin air. You are up at what, 9000 feet or so? You are down to 2/3 of sea level at that elevation in terms of what a given amount of feet per minute of air can carry in heat.
One last thing; and this also relates to elevation. While you do have the turbo, it's so easy to over-fuel diesels at our altitude. Over fueling is the number one cause of diesels making too much heat. What are your EGTs running? You may have to lean out the pump a little to keep the EGTs and overall heat production down.