It should probably be remembered that 'Crew Chief' with the USAF was not necessarily the same as a Crew Chief in the RAF.
A USAF Crew Chief would normally be classified as Ground Crew and look after an A/C on the Ground.
I seriously doubt that a CC would normally replace the Nagivator on long transocean trips.
The B47 could carry 2 extra people sitting in the crawlway/step to the left and under Pilot and Copilot,but they of course would not have a Bang Seat.
No the Crew Chief was not a member of the flight crew on the B-47 but most did at times fly with the plane. Normally this was done when a possibility of the B-47 landing at a air base that normally was not used by B-47s. It also allowed the Crew Chief to be on hazardous duty pay to compensate for the extra duty of traveling with the plane and extra hours maintaining the aircraft as well and the extra pay was helpful as well. To maintain this pay you had to fly at least 4 hours a month, but at times it greatly exceeded that. It was boring duty to fly where you could only see outside by looking straight up as the seat for the 4th crew member was located along side the co-pilots feet just above the cockpit entry door.