My tip for CCE, just use an avisource in gknot process it all you want and up to the right resolution (remember TV overscan will likely be an issue). Take that avisynth and use it for the source in CCE.
A feature of avisynth you want to use is AddBorders()
http://avisynth.org/AddBordersWhich will help you maintain the AR.
I would strongly suggest using gknot to get it to the right AR, then "patch" the tiny bit of resolution you have left with the addborders, matting is necessary for mpeg2.