Here is a link for the 15400 instructions from the Curt website. as noted at the top of the instructions the hitch fits a 2001-present Chevrolet Silverado 3500 HD Cab and Chassis w/34" wide frames. I spoke with Curt yesterday and they verified this.
The dimensions on the Amazon website generally are for the box.
Also per Curt the drop from the top of the hitch mounting surface to the top of the receiver opening is 6 5/16".
I plan on ordering one today and mounting it over the winter.
https://www.curtmfg.com/masterlibrary/15400/installsheet/CM_15400_INS.PDF