Nice and Ugly UDS there Bobbrisket!!! Lookin' Good.
2 inch clearance for ash is the recommended for a typical KISS UDS build but I started at 2in and then added another inch after my 2nd butt. It looks like you're good with the tools so I'd say build it, get it dirty, then adjust as needed. It looks like your top grate is way more than the 24in recommended clearance from the fire box bottom so if you need/want you have plenty of room to raise the firebox to allow for more ash drop/time between sweeping up...
Cookin' on a very ugly drum. But hey, it's simple and make great eats.