Heavy duty engines have by pass hoses like that supposedly for more even heating during warmup.Don't kow if plugging it will make any difference, but try it. Since your truck has a new radiator and you seem to know what you're doing, all the small stuff must be ok.
Now how about the position of the fan,it it low and only covers the bottom part of the radiator since you say it heats up worse in town.And the engine is actually hot,what ever gauge is being used is accurate?And if so what do you consider hot? And the block is not full of rust and sludge?
