Yes, this would work, especially since you will stay with R12. The problem will be mounting the R4 on a Ford. If you are willing to weld up a mount then that can be made to work as well.
That said, it would seem to be much easier and be more original for the Mustang(I am assuming this is an early Mustang) to find mount and drive pieces at the salvage yard for a York Compressor. The Yorks are much cheaper and simple than the R4, and very easy to find, inexpensively, in quality rebuilt form. The Yorks do not convert to 134 well, but work great in an R12 system.
The mount from any old Ford V8 or 6(whatever is in your Mustang) should be easily found at the bone yard along with the idler pulley and drive pulley.
Mixing and matching individual components is no big deal as long as you have adequate condensor size for the application. In the fifties, my Dad pieced together air conditioners from all different components when retrofitting into the cars of the time.
BTW, it sounds like you're moving right along on your TD a/c.