Assuming you only have stock fans, I would leave your front fan in place, and move your rear exhaust to intake in your 5.25" drive bay. Then make sure your top 140mm fan is in the front slot and intaking, and block off the top back slot. If you're willing to spend more on fans, you could add some side panel fans and mid-case fans as well.
A few other things to note, what heatsink are you using? And idle temps mean almost nothing. The only time they might be important is in comparison to your load temps. To get load temps of CPU run something like Prime95 or Intel Burn Test. To get load temps of GPU run Furmark or OCCT.