Hi-Boy website is here:
http://www.hiboydrivein.com/
I grew up on this stuff. My favorite location is a block north of US 40 and Blue Ridge Cutoff. It's the most retro-looking surviving restaurant. That's just about a mile north of Arrowhead.
Another longtime location is US 24 and River, about a 1/4 mile west of the Truman Library.