Unglue the glass, remove the screen, replace the HDD. ifixit has some info on the teardown. External drive either USB3 or TBolt should be just as fast.
Thanks guys. I was actually trying to decide if it would be worth it to get the Fusion drive 1TB or a regular 1TB and upgrade it to a 256GB SSD and have the 1TB as external. Fusion drive price is quite hefty when comparing to regular SSD's + external storage solutions.