1.
Are you sure you have selected FIRST 'Preferences'/'AudioVideo'/'Video Standard' (as NTSC) and THEN you load your input title?
2.
A 1.66 letterboxed to 4:3 DAR DVD should be kept as it is because that does not involve any re-encoding, while going to 16:9 does. Remember that every time you re-encode, you loose quality. I think you can always adjust your plasma screen DAR settings without much pain.
3.
I would not give you a ready-to-copy AviSynth script. I would like you to play a little bit with AviSynth (basic) functions to write a working script by yourself. Don't be lazy, my friend! Just try, I can assure that your satisfaction will be great.
Bye