I have two racks in my 55 gal. with a flat lid. the grates are about 5.5 inches apart. There has only been a couple times I couln't fit the meat in that space. You can also make a tall lid from a donor drum. You have to work on sealing it up and a weber lid would probably be easier.
good luck and make sure to post pics
Crankin out Q on my ECB since 2005
UDS born September, 2007
22" Weber Kettle
18" Weber Kettle