Kitchen cabinets don't take two people to carry a 4' section. As stated before they were professionally made in place just for the tank.
You have 8 2x4's holding up your tank. One 2x4 = 5.25 square inches. x 8 = 42 square inches of support.
I have 90 square inches of support in the two B's alone. 30" x 1.5" thick.
A's are 1x6 solid oak with the center one just split for another 29.6 square inches.
C's and D are 7/8" (I think, I know it's more than 3/4) thick finish quality plywood for another 117 square inches of support.
Bracing that is what the flat pieces you see do. They stop everything from leaning along with the corner craftmanship.
Then the plank that sits on top of all of this is the 3/4" hardi board (I think it's called) that weighs a ton and you could make a ramp out of. That should distribute the weight pretty evenly over the 220+ square inches of support.
And I'll take a good piece of 1x oak over todays 2x's any day of the year for strength.
I'm no engineer either but my math seems to add up ok. Let me know if I am overlooking something. Thanks