Before you proceed make sure that you can upload 900 kb/s because some speed tests are immaculate, if 900 is the speed that your ISP gives you, then you probably won't be able to upload that high.
If you actually have 900 for OBS to work with then, you should use 128 bit AAC audio, and no higher than 700 video bitrate.
You're best off using 360p resolution (you can downscale this in the video tab)
That's what I would do, but figuring out settings is a trial and error process, so it might take some tweaking to get just right.
