Your building setup will be almost identical to mine (42x69x14). My "room" is only 12x12 however.
I have found that the area above the tool room is IDEAL for long term storage. I wouldn't want to guess how many hundreds (thousands) of pounds I have stored up there.
My advice, spend the little bit extra, and make it useful space. You are probably thinking you will have "all the room you would ever need" with this new building... Funny thing so did I. Bought a new to me Kubota yesterday, and barely had space in my building for it. One of my weekend chores is to see if I can clear some additional floor space. Guess where I will store it - yup - on top of my tool room.
Have fun!!