In the Pacific NW. The Mustang 3.7 is very popular. So the "Breakers" (We call them Wrecking Yards) have these engines/trans for sale. Price depends on mileage of the engine. I have thought about a 3.7 but the V8 I presently have, other than MPG, seems the best for Me.
I think I would be happier with a 3.0 rather than a 3.7.