Approx total weight of pan and water with 10% added = 525#
So I will assume that the plate gauge used will be 16,12, or 10.
Unit load on the plate will be approx 0.17 #/ sq in
Now it becomes a matter of choice of how the pan is supported.
The pan should be considered fully supported ( assume bolted to upper frame via a pan lip, thus it's fixed all around.
Using no additional pan supports and 10 GA you'll get a pan deflection of .3" but you can add additional supports ie; put one on the center ( an attached hanger support
running across the bottom of the pan and up the sides into the upper frame.
You can also add 2 supports across the width evenly spaced over the length at say 22" spacing and use 12 GA @ .025" deflection or 16 GA with .16" deflection.
So that gives you an idea of plate thickness with out consideration for corrosion. May want to consider adding a support from legs to center of upper frame.
RICH