I just looked up a 68 Val 273 v8 on www.RockAutoParts.com and they have both the left and right rotors for $ 58.79 each part #2765005L and 2765005R
There you go :)
Bear in mind the left side has left hand threaded studs.
That said nothing says you can't run a right side hub and rotor on the left side.
If you do why not change the left rear studs to right hand threads also.