Um… forget it.
I used some old information I had on an old build.
(Putting this code in the config.txt)
hdmi_force_hotplug=1
hdmi_ignore_edid=0xa5000080
hdmi_group=2
hdmi_mode=82
And it didnt work the first time… BUT
What I found after a very stupid moment is that I had my HDMI cable plugged into the wrong port on the Pi400.
Once I moved my cable to the proper port and booted, I was instantly brought in - in 1080 mode.
I feel so stupid. lol.
Oh well. Hopefully this helps someone else out there.