You should NOT arch your back, and you should NOT roll your pelvis forward.

You want a flat back and a neutral pelvis. Play with your postion (just by controlling your posture) on the bike. Flatten your lower belly muscles (transversus abdominus) and see what happens.

Can't find my tape measure, but looks like 5 1/2 inches is about 140mm. Give a minimum 1 cm margin to each sit bone gives us 160 mm. The B17 is 177mm.

Yeah, I'd bet there's something to play with for your posture, or the nose of the saddle is tilted up too high. Or the saddle is too far back on the seatpost. (slide the saddle forward a cm or two, not yourself, and see how that feels)

Mimi, what do you think? 5 1/2 inch sitbones on a B17? Should be fine, right?