This tank is going to be epic!!!
Would you be offended if I suggest you reinforce the stand with more 2x4s, like this?
This would provide 4 vertical boards that stand UNDER the horizontal boards, providing their compression strength to hold the entire top up.
Without them, only the strength of the screws keeps the top from slipping down the boards.
Without these additional supports, your tank will surely hold several hundred pounds, but with them you would be able to stack about 1500 pounds on it, or more, if you wanted to.
Hope this doesn't offend! I think your aquascape is fantastic and am looking forward to seeing where it goes from here