Back when I was streaming on a 384 upstream, I had my resolution down to 360p@25fps, may have even been lower. Quality was set to 6, bit rate to 210, audio was set to 12kbps 11khz/16bit/mono (although OBS doesn't have any really low bandwidth options, so you have to go with 48). I believe I knocked my encoder preset up a notch too. It's possible to knock up the encoder preset a few notches without a huge hit on processor usage at low resolutions, something to keep in mind. It helps improve your stream quite a bit too.
Most definitely knock audio all the way down to the minimum. AAC makes things sound pretty good even at low bit rates
It's also important to note at such low upstreams you have to set up a QoS profile in windows to limit the absolute maximum bandwidth for a streaming program so it doesn't choke your upload if it spikes. I had to do this at least, but you're in a better position so you probably wont need to do that. You want to give yourself at least 100kbps of room for your actual gaming packets and as a buffer.
