RocketEngineer
Space is big.
How tall is your stand where the tank sits?
In other words, how high is the bottom of your tank from the floor?
My stand is 36" tall. My issue is the AmericanDJ power strips under the top edge of the stand. The 2" they take up is just enough to keep my skimmer cup with a Swabbie from clearing the top of the sump. I have to angle the cup to get it out which can be problematic. Otherwise its a good height.
This will be for a cichlid tank, so i will be using 2 fx6 canister filters.
book shelf section would actually be for storage/pictures/whatnots
AH, in that case using several center supports does allow you to go with all 2X4s. Without those supports, you would need larger lumber for the clear span. Good luck.