Try bolting on the assemblies with solenoids ... The bowing should go away once the playfield is populated. Those drop target banks, flipper assemblies, and other heavier stuff should help it flatten out.

I've disassembled some Bally and Stern playfields that used staples to mount the rails. The playfield was still drilled for screws but none were populated. Maybe they'd staple the perfectly flat ones and screw the curvy ones ? I know I would :-p .

Sorry ... I forgot to add that you might want to see if you missed a screw or two for the rails. If it used staples, maybe switch to screws.