1989 Jeep Wagoneer, 360v8, 727, stock for now,
The fan is for a 1970 Chevelle with 454 big block. GM part # 3976064. It's a aftermarket manufacture.That fan should be plenty large for a 4bt. How did you fit that fan to the Cummins fan hub? The pilot on their hubs is 1" and a GM fan isn't. Do you know the part # on that fan. Many of the fans for the 454 are reverse rotation (counter clockwise). You fan blade is clockwise. Might need to double check that.
As for how I did it see: 4BT fan options