Someone's drawing of a 66 Chevy Avalance
1965 Chevy crew cab
1964 Chevy homemade crew/ext cab
1966 Chevy 1 ton crew cab
3/4 ton Chevy crew cab
Another crew cab
Here is a very different extra cab. It looks like they took part of the bed for the cab.
Here are a couple of webpages that others have created with pictures and information about crew cabs of this era;
1960-66 GMC Crew Cabs - On this page Jolly has information and pictures about Crew Cabs of this era.
1960-66 Chevy Crew Cabs - On this page Van Pershing has some pictures of Chevy Crew Cabs.