I've got a 1969 164 which has probably the same brakes as your car, although
the booster may be slightly different. Does NOT have vented rotors.
My conclusion is that if you use your tranny at all for braking, you don't
need vented rotors (with the possible exception of towing a VERY heavy
trailer on mountain roads with very long downgrades). I have driven this
same car with the same brakes for 33 years now, and the only problems I had
was with rubber brake hoses. I converted to IPD stainless braid hoses, and
redid the master cylinder twice over that time. Last time (1985) I changed
over to DOT5 and it has been fine ever since. I have never experienced
brake fade and I have been in some pretty hairy mountains (close to 12,000
feet) in Costa Rica and Panama.

to say this: I think you could spend your money and effort on better
things than converting to vented rotors. Start off with IPD (or other)
non-rubber brake hoses. The rubber ones swell shut and act like check
valves, keeping the brake applied. Even vented rotors won't keep them
cool in that case!
--
George Downs, Bartlesville, Oklahoma, Central US