AssumeFPS(clip,"ntsc_film",sync_audio=true)

this will change the fps from 25 to 23.976 and sync the audio with the new frame rate. I did this with 9 seasons of seinfeld and it works pretty decent. It sounds like you want to use the ntsc audio though so you could just add killaudio() after this and then use audiodub to mix the ntsc audio with the pal footage. Of course it would be better to encode the video and then mux in the audio later so as to avoid re-encoding but anyway here's how to do it in avisynth:

PAL=mpeg2source(palgout.d2v).assumefps("ntsc_film",sync_audio=true).killaudio()

NTSC=mpeg2source(ntscgout.d2v)

audiodub(PAL,NTSC)