I originally made it for my last truck because the 4.0 took a long time to get heat out of it, i.e. fifteen minutes or more in the real cold air. Once it finally warmed up, it was fine. It was a young engine so it shouldn't have had thermostat issues, but I agree, the radiator shouldn't come into play. However after the block went in I'd have heat in three or four minutes. Its major mileage boost was when used in combination with removal of the fender intake tube. That put the air filter
box's opening directly behind the radiator and in the winter it wasn't unusual to have IAT's 30-40F above ambient going down the road.
This truck has been doing the same to me heat-wise. I hope the grill block speeds things up. I don't get much more than lukewarm heat until I get to work as that heavy iron block is a massive heatsink. Can't do the airbox trick though unfortunately so we'll see if mileage changes. I don't expect much there. After all I got 23.5 mpg last tank bucking snowdrifts in 4x4, how much better can it get in the winter?