You are not going to change the shape in 80 year old wood that has sagged. The only way I can think of to straighten the bow in the ceiling would be to sister new lumber alongside the old 2x4s. For example if you sistered 2×6 lumber to the old 2x4s and notched the bottom corners of the new lumber over the sill plate, it would align with the lowest point of the old 2x4s This would give you a new straight surface to apply your drywall to. Bit costly, but the bow would be gone.