Nevermind, I figured it out.
Enable GPU Scaling and Virtual Super Resolution!
For some poorly thought out reason, AMD does not provide the ability to do custom resolutions. You have to rely on either third-party software to do it, or dig in with regedit to manually add the correct data. I can't recommend any particular approach, as I haven't had a need for custom resolutions in ages. I suggest you do a web search on "amd custom resolution" and do some reading.