Read your owner's manual for your TH50PX75 for:
SDTV/EDTV Aspect Control = Full, JUST, 4:3, Zoom
HDTV Aspect Control = Full, H-Fill, JUST, 4:3, Zoom
As for your STB, suggest you make sure that in setup you select NORMAL for 480i and not WIDE.
Your display is in the native resolution of 768p for all inputted resolutions.
For testing HD (and full screen), you could tune to PBSHD which offers either HD programs or widescreen SD programs and never 4:3.