which psu size

I've recently finished building my very first gaming PC, and I'm looking to buy a ROG Thor 1200W to replace a ex-mining PSU (given from my friend), I'm wondering if it can physically fit into my current rig. (Since the Thor is 1cm longer then the PSU clearance of my case, and I've moved the HDD...

As it was said above, i am planning on buying a new HDD around 250 GB storage, because my old HDD, which is 1 TB, is really slow as it has been formatted many many times. So, my question is: can i buy a HDD with windows 7 installed so i boot my windows with it, while i have my old HDD (which...