Yea your right I was debating on moving up on size for the safety factor. Only Issue I have is that how I designed the tank stand that the widest tank I could slip in from the side would be 18" but that would be hitting the posts.Loose rule of thumb for a sump is about 30% of your tank size. Personally, if I was to do a sump I would aim for a 55g or 75g. The two biggest factors for determining sump size are:
1) How much media (bio, chem, mech) you want
2) How much overflow space you need in case of a power outage.
My biggest concern with a 40g is capacity for overflow, but you can calculate that.
I do not have a sump, been planning one for ages, and recently decided I would convert my 180g to a hmf filter instead. Probably not appropriate for you seeing you do not want the filter in the tank.
Any suggestions on how to fit a bigger one under there and alter my stand?