Very easy to do. Just swap the steering boxes over and remove the pump. 3 bolts hold it to the K-frame and split the pitman arm from the steering linkage.
Although you will also have to loosen the steering column in the car. 3 bolts on the floor and two under the edge of the dash.
The only thing you have to look out for is the size of the output shaft on the manual box (where the pitman arm attatches). These can be two different sizes. This is not a problem if you can't get one to match your original. You would just need a new pitman arm if yours does not fit.
It is also a read bastard to remove a pitman arm from a box. So you might be better off using a new arm anyway to fit on your new box.
Because a manual box is not as long as a power, you will also need to lengthen the steering column to reach it. You can get an adaptor to extend the existing shaft and coupler. Or you can do what I did and use one of these;
http://store.summitracing.com/partdetai ... toview=sku
You would have to completely remove the steering column to fit this part.