Ingame select 1920 x 1200 and get all other options how you want them.
then exit game.
Then go to C:\Users\*YOUR USER NAME*\Documents\My Games\Dead Rising 3\rendersettings.ini
back it up, copy it and rename it, just to be safe.
open with notepad and change the entry in rendersettings.ini to read
ScreenWidth = 1920
ScreenHeight = 1080
save the file. overwrite etc
the p or i etc doesnt need to be entered anywhere.
*THEN* in explorer 'right-click' rendersettings.ini and select properties.
Tick read-only.
Click apply.
Go back into the game *But do not go to video options*
If you go into the video options I believe it will change the resolution for that session.
And dont get hung up on the 'p' in 1080p think of it this way .....
1080p (progressive scan)
1080i (interlaced)
Both the same size both horizontally and vertically.
If these tweaks dont work, just be patient im sure other people are working on it