You can remove your DVD drive and put your hard drive in the DVD drive bay using a 5.25 to 3.5 caddy.
Then take a dremil tool to the hard drive cage and cut it out (or it might possible be removable).
Then you can have a dell OEM case with complete crap cooling for your high end GPU,
Top mounting your new PSU to support the long GPU will not be great either, considering your hot pc air is the intake for the power supply in top mount configuration. And fyi your stock power supply will not support any GPU that requires separate power supply.
You will be much much better off getting a real aftermarket PC case that is designed for gaming. That is assuming that model of dell is not using the proprietary circuit board for...