I worked at a gas station repair shop from 72 to around 78, then went to work at a JC Penny auto service center for a couple years.
I had the extreme privilege's working on a few of the 390 Cougars. When they ran well, they were fun cars. They were a pain to work on. The 390 was shoe horned into them, and the front suspension was a bit light weight for the 390. Those upper control arm bushings were a royal pain, I had to do the grease job on every Mustang and Cougar that came in the door, no one else at either place could seem to get it done, and if it didn't get done, they squawked bad.
If the 351 W is in good shape, and you intend to drive the car a lot, I would not be in too big of a hurry to swap it out. That Cougar is a better balanced car with that 351 W in it then it will be with the 390. The 390 makes it nose heavy. The real life performance isn't much different, and the lighter weight of the 351 W will be easier on the front suspension.
When its time to sell it or it turns into a show poodle, put the 390 in. Between now and then enjoy the 351W.