Ok, Ron, I went back and looked at the drag aspects again.  It looks like the calculation was accurate, however, I think this will put it into a better perspective than before.

The frontal drag at 120 mph for the 1 square foot radiator (using just the frontal area - no drag coefficient) was

37.63 lbf/ft^2,  the "internal skin" drag of the 4" thick radiator was 6.7 lbf/ft^2.  The skin drag for the 1" thick rad was 4.28 lbf/ft^2.  So comparing the 6.7 with the 4.28 was where I came up with the 58% increase in skin drag.

However, adding  the frontal and skin drag factors for the "total" drag, I get 37.62 lbf/ft^2 + 4.28 lbf/ft^2 = 40.98 lbf/ft^2 total drag for the 1" rad. For the 4" rad  37.62 + 6.7 = 44.32 lbf/ft^2, so based on that it appears that the total drag was increased by 41.90/44.32 = 5.5% more total drag for the 4" radiator than for the 1" radiator. It might be a tad bit less than that due to the 5% decrease in mass flow on the frontal area of the thicker rad.

At least that is the way it appears to me.

Ed