at 40g (i'm assuming the typo to be 40g, and not really 400g?), you're looking at a water weight of 320lbs. depending on if you add sand or not, the overall weight will vary a bit, but an estimate of 500lb upper limit is probably ok.
is the tank a long display tank, or a cube?
given the relatively low weight, framing it out of 2x4 would be pretty straight forward, you just use the same staggered pattern of 2x4 at the corners like everyone else's 120gal stands.
since you want a nice display one for your living room, the real challenge will be the finish. what are you envisioning? wood working has varying degrees of finish.
i think it'd help to answer your question / request for help, if we have a better idea of:
1. physical dimension of the tank
2. what your goal is for the stand (link some example images)