bly123,
You didn't mention why you wanted the Mustang motor. If you are getting it for gas mileage, you have made a wise choice over the Windsor.
If you want the Mustang motor just because you want it, then I would suggest you get it in whatever you can fit it into.
But, if you want it to go fast and be "streetable" don't count the Windsor out.
Here is a link that has a bit more detail.
Gobra @ Tulsa
Wonder what the Mustang motors are doing?
J