...the feet of the gunner obstructed the bomb aimer.
I think you'll find that the feet of the gunner were on the end of the Bomb-Aimer's legs.
Except on a 617 Special Lancaster where the displaced mid-upper gunner was sent up front - but they fitted stirrups for that operation.
Lancaster crew - Pilot, Navigator, Engineer, Bomb Aimer, Wireless Operator (generally dual trade WOP/AG just in case), Mid-upper Gunner, Rear Gunner. Seven men altogether and no Front Gunner.