Just an fiy, 4cuft is right at 30 gallons. It might be a little much for a 32 gallon barrel if you want it to fluidize.
what air pump are you going with to move the K1? I have 4cu feet of K1 in a 55 gallon drum and its pretty tight, I would start with 2cu feet get it moving well then add small amounts until it just floats.
good choice i went with the 60 thinking the 100 would be too much, it gets the job done but if I did it over id get the ap100
I'm confused why the stand pipes in the tank only come up a bit over half way.