Exhaust manifolds were the same for all series 2 and 3 front wheel drive 3800's.
The stock manifolds are somewhat restrictive in factory form, however the rest of the exhaust flows fairly well.
F-body 3800's had two different exhaust manifold designs. The later design flowed quite a bit better than the earlier design.
All f-body's utilize 2 upstream oxygen sensors and 1 downstream oxygen sensor.