No interlacing shouldn't matter. Did you make sure to put converttoyv12() in your script? Many filters including chubbyrain2 and gradfun2db/mod are only compatible with YV12 and don't work even with YUV (which is the interleaved equivalent and often the colourspace supplied to Avisynth). If you want to keep 100% quality (i.e. the full luma quality) you could try doing this:

pointresize(width(),height()*2)
converttoyv12()
chubbyrain2() #or gradfun2dbmod()
converttoyv16() #or converttoyuy2()
pointresize(width(),height()/2)

# (edited to fix the linebreaks)

Also, I suggest using AvsPmod instead of VDubMod which is obsolete now. For the credits if they're all black-and-white you could just convert that section to greyscale using greyscale().