post #83 here is painful
https://67-72chevytrucks.com/vboard/...=825785&page=4
but it gives some idea of the difficulty of fitting front sheetmetal around the radiator location of a modern frame. Radiator and engine both have to go back to get the fenders over the wheels.
With a flat deck you get some options about putting the radiator behind the cab to buy some length in the engine bay.