What Rick said. So your first grate is going to be (about) 27" from the bottom of the drum if your charcoal grate is 3" from the bottom. The sides of my basket are 8" high (which seems pretty standard) but I never need to fill it all the way up. I have in the past just testing or when I did a brisket the first time and I could have cooked for 20 hours.
It will be pretty hard to get two grates in unless you do a Weber (or some other tall) lid.