Mix two audio tracks - stretch one to sync with other I have two music tracks of the exact same music (I have rights to it) but each is incomplete.  One is missing the start, the other is missing the end.

Both were recorded from the same source, except due to inconsistencies with the recording hardware at the time, one was recorded just slightly faster than the other and so the overlapping parts are at slightly different speeds and pitches - otherwise they are identical.

I want to merge them together, effectively overlap them, into one complete version of the song, but need to adjust the speed and pitch of one to match the other.  (Then I can simply line up the overlapping parts and fade one in and the other out to get the result - good enough).

Of course, computers can adjust pitch and speed independently by applying clever algorithms, etc. (although sometimes this results in some quality loss) - unlike say speeding up or slowing down an analog tape or record player where changing the speed changes the pitch as well. This "non-clever" analog-type outcome is fine - in fact preferred (to avoid that potential quality loss) - because knowing that the original source for both tracks is the same, getting the speed right this way should get the pitch right anyway, and vice versa.

What app, if any, on Mac can help me achieve this (stretch/shrink an entire track uniformly, both pitch and speed, to sync with another), and what functionality or steps or whatever do I need to understand to do it?  

I've tried GarageBand, but if that can do it, I can't figure out how. I'm probably missing something.  Or perhaps GB isn't the app for this...?  Perhaps Logic Pro can do this, but this project isn't worth the cost of that app, and surely some free or inexpensive app can do just this (simple?) task?

Any advice/direction/help most welcome. Thanks!
I suggest you try Audacity, which has very powerful audio editing tools, including the duration stretch/compress and pitch change functions. I've used it many times for a variety of audio tasks over the past 10 years or so. I cannot recommend it more highly. Catalina is not currently supported, however.

If you need to save to .mp3 and other formats, you will need the FFmpeg library (links on download page) for these functions.