The TV aspect ratio options are on Television options window. The default settings normally should be the best, i.e. "Source Aspect Ratio" and "Crop Edges to remove visual noise...". If you do not want letterbox, you can try "Stretch" or "Crop" as your aspect ratio option.
While playing a channel, you can use Up or down arrows to make changes with On Screen Display menu.
You do not need to scan channels for each tuner separately. MC will pick an available tuner to use. You do, however, need to scan different types of channels separately. In your case, analog channels and QAM channels should be scanned separately.