Apevia 500w SFX power supply (Amazon)
This Apevia 500W SFX PSU is the reason this build works, because you need SFX sizing for the Optiplex 7040 case. It was only about $35, which is wild for an SFX unit, and it gave me enough headroom to run the 6650 XT after the Dell PSU swap.
You'll be taken to Geni to complete your purchase.