The stand looks well built and quite strong.
I only have a tiny bit of experience with a situation like that. I built a stand to hold 2 40 breeders (one above the other. It had flat boards on top of the structural beams. The top board warped a little and I didn't notice. Later that year, the top tank cracked (front panel), making a mess.
The wet wood of your stand could be an issue (I am not sure), as it certainly could change shape (unevenly) and that would not be good. Perhaps others have more experience and would chime in.
It it were me, I would put a large amount of weight on it (such as a large empty aquarium, perhaps even heavier), making sure that it is all evenly distributed, and shimming it to conform to any floor irregularities. I will NOT fill-up the aquarium. I would place the whole thing (stand + weight above) in a room with a dehumidifier running 24/7 for a good while; I don't know how much it would take, but it could be weeks. Only when it seems that it is fairly dried-out would I consider putting it to full use, such as filling up the tank.
I guess you may be having a delay in putting together your new tank....