As I said in post #6, I had already bought the replacement unit (which both cools and heats) and it is installed and working.

The old unit was installed in the wall under the only window.

I figured that since I would probably end up with a bigger unit, I would have to install it in the window and fill in and cover up the hole. While the new unit has more capacity, it is the same physical size - simply serendipity - and so it went back into the exiting opening. It was the path of least resistance. I still have some trim carpentry and caulking to finish, but all is well.