Where are you located? This would obviously be a custom build job, so the tank manufacturer should be able to make it structurally sound for you. I think those dimensions would be a lot easier than 4' by 6" (length doesn't matter here for maintenance purposes). It will make it easier to get any kind of decorations / plants in there too, without them brushing the glass on each side. That has the potential to be a very cool, unique tank - if you do it, I'd love to see pictures.
Before ordering one, it would be a good idea to grab a poster board or roll of paper or something (even a shower curtain) to draw the rough sketch on, and stick right to the wall in the intended place. Then you can pretend to reach into it and see if there are any other modifications to shape or locations, before creating a hole or spending money to have it built. Also stand back and look at it, draw some intended decor, that kind of thing. Try to figure out where heater, filter, cords, etc. will be, so that you have electricity available at the right locations. It is a lot easier to draw it out, and replan ahead of time as needed, than to have it sitting in a hole in the wall, and thinking "huh, how do I get this cord to an outlet now." I'm assuming you'd want to have the electric setup in the wall if possible (lighting too, unless you're wanting to go with just ambient lighting). Actually, I saw some "under the rim" lighting at Petsmart recently, which might work well for that kind of build.