A high output in a 1986 Ford pickup truck?
Keep dreaming. It's probably just a standard 351W with normal compression (8.7:1 typically) and a 4-bbl carburetor. Seeing whether it's carbureted or not is going to be key to identifying whether it's truly a 351 or a 302. 302s were fuel injected that year. Another thing: are you certain it's the original engine in this vehicle?
There's really not too much to be gained installing this in an '84 Mustang unless it runs absolutely perfectly, and with very low mileage. Granted you will most likely see more torque, but unless you plan on adding some parts to make it use all of its cubic inches, I'd save myself the trouble. It's not too difficult to install a 351 into a Fox body, but it will sit higher in the engine bay, which will necessitate a cowl hood or special engine mounts. Also, the added torque of a 351 (which is a real kick in the pants) will stress teh drivetrain considerably. Therefore you may have to upgrade the transmission and rear end. To summarize, yes it will work but with caveats.
As to an actual horsepower measurement? Hmm....I believe it's probably around 170 for that year. Truck motors are made to make low-end torque, not so much in the horsepower department.