You could use BeSweet as Moth3r recommends and get a difference on the number of frames. Then do a time compress/expand of the audio in a higher-end app like Vegas.

Once you have the number of frames for the target, I would load the AC3 into Womble and export as a WAV file. Then load that WAV into Vegas, normalize the audio, perform the stretch/squeeze to the number of frames, and export back to AC3. Womble is a great way-station for AC3-to-WAV conversions, where other tools I've used can cause changes in the length or introduce digital pops.