A collar system for a connecting the cab of a vehicle to the cargo bed cover or similar boxed attachment, the vehicle mounting collar including a webbed collar frame designed to mount to a standard driver's compartment or cab, without modification to the vehicle. The collar and attachment method are extremely flexible and accommodates any vehicle tolerance variation. The collar mounts to the rear flange on the cab of the vehicle, and the bulkhead of the collar has slots to receive the shell of the cargo bed cover. Shell bolts within the bulkhead slots allow the shell to be adjustably repositionable on the bulkhead of the collar by slippage of the shell bolts within the bulkhead slots.