You start with a digital copy of the movie I'm starting from...

I extract the audio...

And then my script downloads the Public Domain movies...

...and I've got a hard-coded list of cuts to take from each of the Public Domain movies...

...and combine it with the movie audio?

My script is just a few hundred KB.

As long as the movie you start from is pretty well in audio synch with the version I start from...

Heck, I could probably even compensate for that, too... A little bit of analysis to listen for the first words spoken...