The 40 could work.
Besides equipment and whatever media you may want to fit in it,
how much water is in the last(resevior) chamber is important IMO
Depending on design(baffles) you could end up with less then 10g in the last chamber and find evaporation to be an issue that would draw air into the return pump without a auto top off system in days?
I use a 30 b on my 120 fowlr
with no overflows,and still lose 5 g every 3-5 days depending on weather.