What he said^. Try arranging the plug wires using the HO firing order. If it runs like dog poo, then it is a non-HO 5.0. Based on how those pulleys look, that came from a passenger car that was not a mustang, so I doubt that is an HO engine, unless it came from an 86' lincoln mark 7 LSC (not sure how their pulleys look though). The 86' HO mustang accessory pulleys look like the 87-93 HO mustang pulleys. I would look for an 87'-93' HO engine and sell that one. 87'-88' 5.0 HO's were the best (not including 93' 5.0 cobra engine) since they had a slightly more aggressive stock cam, compared to the 89'-95' 5.0 HO engines. But any 87-93' 5.0 HO engine would be a decent choice. Oh, and beginning in 93' they used hypereutectic pistons instead of forged pistons. Technically the forged ones are stronger, but for an non boosted application, it shouldn't cause any problems. If anything, they are better for naturally aspirated applications since they are lighter in weight and expand more quickly to avoid any slight blow-by during cold engine operation.