Found this on craigslist, is it rare?

[FoxChasis]

351W is not exclusive to the Torino, and all my books do say that it is a 351W cam, but from a 351W marine engine.

Didn't say the 351W was exclusive to the Torino.

Any book that says the original H.O. 5.0L cam was from a marine engine is wrong.

The cam that was in the '82-'84 H.O. 5.0, and the '85 CFI H.O. 5.0L was originally designed for a 1973 Torino w/ a 351W, hence the engineering number of D3OE-6250-AA.

From The Official 1979 Through 1993 Ford Mustang 5.0 Technical Reference & Performance Handbook:

The 5-liter V-8's first aggressive camshaft grind debuted for 1982, a design previously used in a 351 cid (5.8L) Windsor two-barrel (1973 Torino) passenger car V-8. (The marine cam grind originally reported to have been selected for this 5-liter combination was actually rejected for production. Although the boatmotor cam offered excellent horsepower potential, it was found to be down on torque.)

From Jan. 1994 Muscle Mustangs & Fast Fords magazine article "Talkin' Engines with 'V8 Wally' Beaber":

Jim McGraw: But you were most closely asociated with the V8 engine in the current Fox Mustang. Were you there at the beginning?

Wally Beaber: Yes. In 1979, when we introduced the Fox Mustang, the 5-liter was released, but we didn't tell anybody about it. We were hyping the turbo 4 and the Cologne 2.8 liter V6. Then in 1980, we brought in the 4.2 V8 with automatic only for two years and dropped the 5-liter. We were going to drop the 5-liter and replace it with the 3.8L V6. The 2.3 turbo had some in-use problems with emissions, and we had to take it out of production. When that happened, the company took a look and said, "Hey, we have a Mustang out there and no performance option!"

They asked my manager, Jim Clarke (father of the modern 5-liter and later of the modular engine program), to put the 5-liter back in. Jim came to me and said, "We’re gonna put the 5-liter back in, but, gee, we ought to warm it up a little bit." He wanted to put in the marine cam, the 4-barrel cam, for power. I thought, We’ve got an old Torino cam that will work better. So we did some dyno tests and we used the Torino cam in production. Even with a 2-barrel carburetor it went 132 miles an hour in the Michigan police car trials, and we didn't better that for about two years with the 4-barrel.

We were revitalized. We opened up the air intake with a dual-snorkel air filter housing, a bigger filter. We worked on traction because it had terrible axle hop at first. The next year we put on the 4-barrel and did some more tune-ups. We had gradual improvements every year through 1987, which is when we stagnated. In 1988 the new Mustang was going to be a Probe with a Mustang badge. The customers wrote in thousands of cards and letters and said, "We don't want that."