Heya
I've done a drawing for you, look here.
http://imgur.com/NbJrHcI
It includes the size of your PC case, the Radeon R9 390, and the GTX 960.
I hope I've got the dimensions correct in my drawing.
The GTX 960 is only
4mm smaller than the R9 390.
Which is about half a fingernail haha.
You'll have to decide for youself if it will fit or not, but if my drawing is correct you should be able to tell quite easily. I've drawn everything to scale.
Here's where I found the size of the two graphics cards:
Radeon R9 390
GTX 960