Dell OptiPlex's are designed as server/workstation machines and as such were designed to accommodate little/no modification by the end user. Sadly their
service manual state(page 18-19) that you can not drop in a higher wattage unit, simply because there are none in that form factor.
The certified manufacturer of such units only make 265W at best but since it's been a long while, finding a reliable brand new unit will be hard to come by ata reasonable cost.