audio and video 23.976fps to 25fps using mp4

[mizumi]

I would like to know how to deal with a NTSC source I want to convert to PAL to synchronize it with a PAL audio track I want to add to it.

The video is mp4 (x264) and I use Avisynth & MeGUI for editing the video and Adobe Audition for the audio but I'm open to use other programs.

This is what I would do:
1. Convert the original NTSC source to 23.976fps using IVTC.
2. Convert the video from 23.976fps to 25fps using AssumeFPS("25") in avisynth or some other program? I used to use virtualdub's function but with mp4 it's not possible
3. I have to modify the audio lenght but don't know how to do it.

My doubts are these:
a) Do I convert the video from 23.976 to 25fps or leave it at 23.976fps?
b) Do I have to increase the lenght of the NTSC audio and modify the NTSC video from 23.976fps to 25fps? Or do I have to shorten the lenght of the PAL audio to be the same as the NTSC audio?
c) How do I the pitch correction in the voice when converting the audio?
d) In case I cannot do IVTC and have to leave the video at 29.97fps, how can I synchronize the original audio with the PAL track?