The actual "H.O. 351" was phased out in '87. It was a stock block with a bigger cam, and a holley 4bb carb, they also put better heads on it. I have a '92 E150 with a 351 H.D., which is similar to the HO, the throttle body is ~56mm instead of 49 or 51mm (I forget the stock size) and the upper intake runners are a little longer.