Windsor ???

Yes,

the last year or so of production of the GTS, GT5 & GT5-S models, circa 1989 - 1990, had 351W motors installed. The switch was made for one of two possible reasons: DeTomaso's Australian source of motors apparently ran out of Cleveland motors, or else DeTomaso re-established ties with US Ford while making preparation to produce the Pantera Si, which utilized a US made 5.0 fuel injected motor.

your friend on the DTBB

George is correct. Nobody is exactly sure when they cut in the Windsors (well, let me rephrase that. *I* don't know when they did, nor has anyone been able to tell me, including DeTomaso!)

However, apart from historical value, they aren't a great motor. I personally know someone who bought an '89 GT5-S direct from the factory and paid the supplement for the "sport" engine option. It was supposed to include a different cam, heads, etc. Upon engine tear-down in 2000, the internals revealed that it was a bone-stock truck engine.

I guess what I'm saying is that I'm a little skeptical of "Tuned for Pantera". It would be fine as a replacement motor if you are looking for a crate engine, but is nothing special and I would only consider it if I got a really good price on it.

I'd heard that the supply of Aussie Clevelands dried up, which is why they switched, but I would classify that as an unconfirmed rumor.