If the masonry is gone and there is just a framed out hole then galvanized would totally work with fairly easy install with the help of a second person or some clever use of support. You wouldn't want to just blow air through the cavity without ducting as it would be horribly inefficient and tank the air velocity across that much distance.
Adding an in-line towards the middle does mean having controls up in the attic more involved and you do want the Mrs to have control over the fan there instead of her having to go downstairs.
If you wanted to circulate the air instead of forcing air with a passive return, then you get into a more complex situation with two ducts and fairly informed placement of intake and return.
Pulling into the attic will help to lower temps and will be the simplest/cheapest option so long as there isn't a wall you can put a window heat pump unit in.