While I have things out for my Motronic to Megasquirt conversion and hydraulic brake assist to standard vacuum, I thought I'd take some pictures of the holes from a factory AC installation on a 1972 e9, which should be the same for most e9s.
The picture of the right side firewall show the holes that were used for the red braided cloth hoses (one thick, one thinner) to the evaporator inside the car. I combined both holes together to make one slightly oval hole, now filled with the Motronic wiring harness. The picture in the lower front splash wall shows the oval crimped opening where the two red hoses go to the condenser.
Luis, you should also have a mounting bracket for the AC drier that would help you with those three exact hole locations.