Hey Marc,
Congats again on the new tank. I'd recommend the 3/4" plywood top also. The foam is a different story though. I used Styrofoam under my tank but only inside the trim area and of a thickness less than the gap so it doesn't actually rest on it. I placed it there more for insulation purposes. One problem with large tanks with bottom trim and foam is that the edges compress leaving the large area of the bottom to create pressure on the bottom glass panel. A large flat area of foam will not compress easily and can actually due harm. Of course, if your new tank is of trimless design, then never mind what I just mentioned.
Either way, it's always better to fabricate the stand properly and ensure that it is perfectly flat and level. The plywood should fix any minute imperfections.
How do you plan to handle the construction of the corner of the upper wall? Is it not a load bearing wall? Will you be utilizing a header beam?
Joseph