Hmmmm, guns in bombers should not suffer convergence effects. Maybe a bit but we're talking inches of separation, not feet like the spread in fighters. You shouldn't think of multiple bombers having a convergence effect the same as a single fighter.
I suspect the multi gun positions like the tail gunner, ball turret, etc, were not set for convergence. They were likely aimed out to infinity in parallel. If one gun hits then the other is only a foot or two away.