A/C in non A/C car

We are putting after market a/c in my dads '72 but wanting to run it through the factory vents. It's a non a/c car and we can get most of the vent parts, BUT the vent hose connection backing plates. The plates are rectangle front side like the vents are and round on the other side the size of the hoses. I might be wrong on these but thats what i seen on other a/c units. Any information on these (description if i'm not right) and where to get these would be a great help. Tried to find some from anybody changing over to after market vents or removing a/c but having no luck. Please Help.