If you look about mid way down the page of the link below you'll find an option called "MonitorLayout" which I believe would be placed in the 'Section "Monitor"' of the /etc/X11/xorg.conf file. Based on how I understand that option, you would add a line to the monitor section called Monitor Layout to use the primary port (head) as in the sample below:
Identifier "Dell 1905FP"
HorizSync 28.0 - 64.0
VertRefresh 43.0 - 60.0
Option "MonitorLayout" "TMDS on primary"
My Dell 1905FP and nvidia card have DVI, but I don't have that option specified, probably because it was hooked up via DVI during installation.