You guys have talked me into going ahead with the conversion. Either that, or maybe I just want to buy new parts for MY bike, I don't know...
The EF51s I have are definitely the two-finger version, and are linear pull only. If I replace them with similar EF51s (or the EF65s noted earlier), I will most certainly buy the four-finger version, so I can use them with either style of brake.
Notably, I was moving all of this from a larger frame bike (my 19.5" Motiv Stonegrinder) to her 13" frame Trek 850, so all of the existing cabling (which was all fairly new already) was plenty long, even after snipping off the frayed ends from the old cable crimps. I have three of the systems connected and tuned (front brake, FD, and RD), and have a black single 31.8mm Origin8 cable stop ordered, along with a new KMC chain. Once those come in, I'll be able to set up the rear brake and have the drivetrain running smooth.
Thanks to all for the advice.