Fit a door in a tight space
Checking online there are the actual unit dimensions for these doors on the "Big Box" websites. I am finding the actual sizes are 81 5/8" in many cases. Tight for your opening but a little off the header will get it in. As a note, this is for an exterior door. Typically interior doors are perhaps a 1/2 shorter. If the header needs a 1/4" taken out of the bottom, and you have a 2x6 header for a 36" rough opening, there will be no issue. The sheathing on the wall outside aids in supporting the surrounding framing.