Thanks for the replies Kronos8! This gives me a lot to think about and some ideas.
I also managed to find, pretty sure, what the fan hole spacing is. I found a guy using an XSPC Rasa360 radiator
which best I can tell uses 15mm fan hole spacing for 120mm fans. I noticed the fan holes lined up with those 2 "tabs" going across the large hole in the top. Not sure though if the 360 rad completely covers up the hole or not - I'll be doing some measuring a little later.
I wouldn't mind putting a 420 in there - the first really good mod I found pictures of here was doing that. Unfortunately he had to Tig weld some tabs in place to provide places for the top fan mounting. At least the way he did it. I'll have to look and see what I have for case cutting etc. We moved recently and a lot of my gear is in storage (which means I can't find it, which box is it in, etc., LOL).