I tried the usual /etc/X11/xorg.conf and it crashed, badly, although that's where I had the modified xorg.conf file in Mint 18.0 and it worked fine. I didn't try shifting it to /etc/X11/xorg.conf.d, maybe that would have worked.

By default, Mint doesn't have an xorg.conf file now, so it needs to be created to make changes like this, and I just replicated the copy of the file I had from when I modded Mint 18.0 to work with the same display resolution. I'll have a look and see if creating xorg.conf in the next folder down gets it to work, but right now it seems happy enough just running the xrandr commands during initialisation.