Hello, love me some 4 eyed mustang. I owned a 85 coupe for a bit. The block is stamped and the number can typically be found under the starter. You are probably going to have to remove the starter, I know I did on mine.
The build sheet of the car should be under the carpet, typically they are under the drivers seat or in the hatch area under the carpet as well. If this is a vehicle you are looking at buying you might have to talk the owner into letting you pull the seat to remove the carpet. Even then there is still no gurentee it will be there.
Another thing for you to do is pull the bucktag. It typically will be behind the driver headlamp and IIRC will be screwed in. The information should be stamped on that tag with options the car came with.
Here's a pic for refrence on the buck tag:
Good luck and let us know what you find out!
Hope that helps.