What your measurements indicate is a 1.5 DIN. It's a little taller than the "normal" single DIN chassis but not a full double DIN. It's probably the least common size but their out there. It's what a lot of Chevy's from that era used. Crutchfield can sell you an adapter to put in a single DIN radio which are very common and cheap. Or, you can look around for a 1.5 DIN and get a little bit larger display and controls. That's what I did in my '05 Chevy truck. I have a JVC unit and I really like the size.
If Crutchfield doesn't know the answer than nobody does. Maybe you got a new guy that didn't know.