Fitting audio from one film source to another film source isn't difficult, just time consuming because you have to scan through both sources at the same time to make sure the frames sync between them, removing or adding frames to the one with the audio so it matches the one you're using the video from, if the playback speeds are different it doesn't matter so long as neither of them have 3:2 pulldown applied (must be removed beforehand).

Speeding up the audio for PAL is very easy, either use something like CoolEdit to resample it or when encoding to AC3, BeSweet can do it on-the-fly.