What do you mean about corner braces and center brace getting in the way under your stand?
Space in my stand is about 14" x30" under my 65g, with a center brace on the front and back (between the doors), but I fit a 15g (12x24) by placing it through the top of the stand before placing the tank on. Not sure if that's the problem your having though.
For what it's worth, I don't think a 40g (that's 3 feet right?) needs a center support in the middle of the front and back horizontal beam. If you make those beams beefy enough (especially if you use a real wood like poplar instead of HD 2x4s) then they'll have no problem giving you an open front without a vertical support in the way.
I'm a furniture designer, so if you want to run your plans by me, I might be able to give you some advice. Also there are a few people here who have experience building a number of tank stands, and they could probably be helpful as well.
Finally, if you decide you want to make a sump, I don't think you'd have trouble. If you have the equipment to build the stand, you probably have everything you need for a sump (especially if you have a jointer). Only problem is that materials arent' cheap.
