So I am now thoroughly confused. Last time I tried put the tank up, I had the corners touching but nothing else. Ok, but not great. So I got a dark thread and pulled it tight across the surface, corner to corner on each side, and sure enough the string only touched at the edges. So I sanded, and sanded, and sanded, until I had all the edges and diagonals with no more than 1/32" gap, between the string and the surface. I thought, great, that is good enough!
I check the bottom of my 55g that I have been using as a reference for the long side since I cannot lift the 120g. It is slightly concave up (gap is still between the string and the bottom of the tank along the entire length.
The only place they touch is in the center with about 1/8" gap under each corner. I do not understand. Perhaps the 120g is bent a lot more and it will be fine. My friend is stopping by tonight to help me try it out...I tested the bottom of the 120 against a yard stick and it looked about the same as the 55, so I am not hopeful. Taking after mudbuttjones, quikwood and 1/4" thick wood strips have been purchased. I think I am just going to use the quikwood on the corners and center and make a putty out of wood glue and sanding dust for the rest just to make sure that those 6 points are good, which will be more than enough piece of mind for me.