If you are going to be pulling a box trailer, I don't think you will want an overdrive. GMC 3/4 tons that were born with a 270 and 4.56 ran 60mph+ all day long back in their day. The speed limit was 60 in Texas until 1962 when it was raised to 70 and we used to get speeding tickets in stock GMC pickups. And it didn't hurt the engines a bit. The 55 speed limit came about in the mid 70's to save gas, but the vehicles were built to go faster than that.
