some places, like the bottom, are not on the drawings or pictures at all. I guess that the rivits will go around the edge, where they would be needed to fasten the bottom to the internal angle irons.
In a case like this I copy a row of rivits from somewhere on the tank that the drawings or photos do show me, using the same spacing. Then I try copying from a different set of rivits. I decide to go with the smaller rivits, since the bottom is only 1/4 inch thick. Somewhere I've seen a table that shows the standard rivit sizes and spacing for different size plate... I have to see if I can find that again.