Mac,
I don't think that you can do that.
On an MBR disk the partition length and partition start addresses (in sectors, hard limited to 512B / sector) are stored as 32-bit quantities, so it isn't possible to address the start of a partition beyond 2TB, therefore using MBR the disk "appears" to have a max total size of 2TB* - it can't "see" beyond that number of sectors.
* actually 2TB - 512B
Unless you are creating a RAID array, I think that your 8TB is still (just) theoretical. I'm not aware of a single 8TB disk being available (yet), the largest currently is 4 - as I'm sure you are aware.
I have tested in the past just to see what happens with a RAID array and single logical disk > 2 TB (2.5TB - 6 x 500 GB disks RAID5) with Windows Server 2008 installation & partitioning - I don't recall exactly what was shown at the partitioning screen, but I do recall that it was not possible to create a partition > 2TB, nor was it possible to "see" the unused space after creating a partition with the maximum allowable size.
SD