Quevvy, let's see what we can do for you here.
Are you familiar with the backup software Macrium Reflect? That tool can be used for cloning your Windows 8.1 partition to the drive beside of Windows 7. Download & install from here. When creating bootable media, I suggest you create the WinPE media, it's more flexible, but will require the download of some files.
However first, some things has to be done. First off, create disk images of what you have for safety, this can be done with Macrium or the backup tool you've been using. And be sure to have install media for Windows 8.1, or a repair one.
Then you want to defrag Windows 7, really good, like twice, Using the built in Disk Management tool, you can shrink that partition to fit Windows 8.1. And do the same with Windows 8.1, defrag & shrink. You will have to run a good Disk Cleanup & delete some restore points, then work on the shrink. It may be better to go with 350GB with Windows 7 & leave the rest of the space on that drive for 8.1.
Once you have both OS's shrank, make sure that both are bootable, if so fine. Now boot into the Macrium media you created, what you want to do is Clone the Windows 8.1 partition beside of the Windows 7 one. You'll click onto that partition, the options to Image & Clone this partition will be shown. Choose Clone. Then select the Destination. That's beside of Windows 7. If there's not enough space, Macrium can adjust it a bit (not to worry, I cloned a 1TB HDD to a 120GB SSD with the tool). Click Finish & the Clone will begin. This will take a bit, but not real long. Once finished, disconnect the drive that you cloned from & then boot the computer. Hopefully Windows 7 & 8.1 will boot (I know Windows 7 will). If by chance Windows 8.1 won't boot, this is where your install media or Repair media comes to use.
Once both OS's are booting fine & there are no problems, shut down & replug the drive back in. Now you're ready to install Linux MInt 17 to the 2nd HDD, but there won't be any drive letters, Linux doesn't work that way. Are you sure you want to dedicate a whole 500GB HDD to LInux MInt? I ask because I have a 120GB SSD & am barely using 20GB of it, have a lot of software installed too.
This is my suggestion. Using any bootable partition tool (GParted from Live Linux) is fine, format & delete the Windows 8.1 partition. Create yourself a nice 200GB Data Partition as NTFS at the right end of the HDD (the slower portion). Then use the rest for your Linux Mint install. Create a 25GB (25600MB) partition as you main or "/" partition from the drop down list, format as ext4 & be sure the Format box is checked. Doesn't matter if it's Primary or Logical. Than create your Swap partition, at the end of the empty space, of about 4GB (4096MB), the Swap won't be formatted, just marked off. Create the rest as /home from the drop down list, choose ext4, and again, be sure the Format option is checked.
Once this is done, click next, the things will begin to happen. You'll create a username (the actual one will be lowercase), a login password (accept the defaults, don't encrypt your Home folder), then you'll be asked to test your keyboard, select time zone & the rest will take care of itself. When complete, you'll be asked to remove the media & press Enter, do so. The computer will reboot, update the OS & enjoy.
BTW, I have 3 triple boot computers the way yours will be, only mine are on SSD's, the main one all three OS's. the other two, Windows on SSD, LInux MInt 17 on HDD.