Adding fresh air vents

How hard is it to add the fresh air vents?

My car is a 74 duster with ac and it had no vents. I got the heater box from a none AC 74 and it had the fresh air vents.

I know my car doesn't have the holes cut in the cowl to let the air flow but is there any other issues with this?

I didn't want to go just cutting holes and then find out there was a difference in the cowl and now i'm s.o.l

Thanks for any help