When was the last time you experienced "brake line failure" in any vehicle, let alone a coddled classic? My guess is never. I'm not saying it doesn't happen, I'm just saying it's rare enough not to worry about. (this assumes that you're like the rest of us who checks under the hood all the time and notices the slightest change in performance of any system and deals with it appropriately).
Every vehicle I own with the exception of the F-Series pickups has a single reservoir master cylinder. I have never owned a modern car. I have never experienced brake trouble on the road due to a brake line failure or any other reason - including anything associated with a single reservoir m/c.
And no, I'm pretty sure there isn't an under-the-floor bolt-in replacement. You'd have to mount one on the firewall, re-jigger your pedal, etc.
Spend your time and money on seat belts.
