I am doing the opposite, i.e. adding a/c to a non-a/c car, a 1970 Chevelle Concours wagon. I have to cut in a new hole through the fire wall that fits the a/c-heater box. It is oriented vertically, where the "heat only box" is oriented horizontally. I would suggest that you make a template from a car with a heat only box, for all of the holes. The heater core is different, so it will require replacement. What will you do with the dash? If you're trying to make it close to original, the '70 non-a/c cars had no holes for the vents.
Let me know if I can be of further help.
Mark T. in L.A.
'70 Chevelle Concourse wagon.