I think its pointless to have the SD feed. Its wasting bandwith that could go over to the other feeds. If its for people that are still watching analog televisions. the converter box will show the HD feed in the letterbox format so you won't have to worry about any of the picture getting cut off unless someone is using the zoom feature on the box. The SD feed is also sending out in letterbox too, so it won't make any difference whether you watch 29.1 or 29.2 on an analog TV, they will look the same on the screen. On a widescreen TV MPB .1 will look windowboxed with all four sides with black bars.
If the sides of the picture are getting cut off its the cable companies fault.
a pet peeve of mine is the way most televison is shot these days. Because of all these people on cable and who don't have their sets adjusted right or use the zoom button, its spoiling it for us HD viewers. The local stations and networks put their on screen graphics and screen clutter too high up and in the center of the screen so they won't get cut off on cable and those idiot zoom-button people. They also cheat us out of a true widescreen picture by backing the camera up and framing everyone is in the middle of the frame. A group, or two shot makes everyone too far away from the camera and nobody on the sides. my rant for the day