Coincidently, my system is chugging away on virtually the same kind of job you're asking about as I type this. Only minor difference is starting out with 720p and delivering at a somewhat higher bit rate.
So:
Choose the MPEG program stream. In the Video Foramt of Encoder pane change automatic to NTSC and the Aspect Ratio will chage to 4:3.
Under the Quality tab, it may show the bit rate slider only going down to 10 Mbs. If that's the case, close the tab, click outside the job – somewhere in the Batch window, re-select the job and re-open the Quality tab. Should now have the choice of selecting bit rates< 10 Mbs. Check under Rxtras and yo should see Stream Usage as Generic and the Multiplex box xhaecked.
Good luck.
Russ