My top Rack is 1/2" from the top, 2nd rack is 5.5" from the 1st rack. I Used the weber kettle lid. You MUST use the regular drum measuring 22 9/16" wide for the kettle lid to fit perfect. It's important that if you build a UDS with a weber Kellttle lid, to have the lid first. I ordered mine directly from weber for $52. That way you can take your lid with you to make sure it fits the drum snug. Rolled top lids will not work to get the tight seal necessary to control airflow.
I cut the tops out with a plasma cutter
Brisket on top rack
Beer can chicken on the bottom
No diffuser plate and I cooks even all around, no hot spots
I have built many different configurations of UDS's and this one is definitely the most efficient and best cooking one. Good luck on your build.