 Re: 5.0L vs 5.0L "HO"
I noticed that you mentioned 1986. The reason I mention this is; the 1986 5.0L Mustangs were very unique. It was the first year for fuel injection. It was the last year for forged pistons. The pistons are flat topped. The heads were a one year production style that were suppost to swirl into the ports, but didn't work well in reality. The 86' look identical to the 85' except that the 85' was carb'd. Also the 86' was the first year for the US mandated third brake light. 1986 is considered a bad year to modify. First, the pistons were flat topped, so putting a cam into the engine is very limited due to valve clearance. Most aftermarket heads have a hard time being bolted on for the same reason. And finally, the computer was Air Density controlled, rather than Mass Flow Sensor controlled and has limitations as to the perameters in which it will allow modified parts to be installed. All is not lost, since you can easily get a later year engine from a junk yard for pretty cheap, that will bolt right in and eliminate these problems.
In regards to the H.O. designation. The H.O. was unique to the 5.0L put into Mustangs. It was the only V8 engine put into Mustangs. It is abbreviates High Output. If it is a Mustang with a factory V8, it is a H.O.
There is a big difference between the 5.0L engine put into Trucks, Crown Vic's, etc., compared to the 5.0L H.O. engine put into Mustangs. As Jarrett outlined, the H.O. is a much better engine in almost every respect. | 
