This all sounds like an excellent idea but my RP, with what I believe has 'standard' brakes has ball and cup joints at the rear wheels.

If I were to change to a clevis at both ends arrangement which rear brake levers should I be looking for?

Turnbuckles are generally called bottle screws by the boating fraternity. All sorts and sizes are available but I doubt one could find something with wide enough jaws to fit directly onto an existing clevis unless it was quite big. Mind you I haven't looked.

Also...although they have locking nuts they have a habit of unscrewing (on my boats anyway) and are best wired to prevent this.