I bought 2 - GMC 5500 dumps with 11 FT boxes, needed a 2nd truck as my F550 was not enough to do multiple jobs, like the 5500s so well that i purchased a 4500 with 8 FT box for lawn crew. All 3 are Duramax diesel with 6 speed tranny. We're happy, but our GMC dealer's service department is 10 times that of our local Ford dealers. I think the GMC trucks are built with heavier springs than the Ford F550 was, but both are rated at nearly the same weight. As for the 8.1...we have it in 2 pickups & i wouldn't think of putting that motor in a 4500 or 5500....the gas u would use......WOW!