AC heater box to non ac heater box 68 barracuda

First, ignore the sharpie markings on this firwall. Those are intended for the a/c addition changes planned for this car.
So the framing square wasn't going to work no way no how. While trying to position it I noticed a < notch cut at the bulkhead connector opening. So I pulled a string from there to across the center of a hole at the far right. I'll assume yours has these same reference points. The string dissected the center of one of the holes around the fan motor flange so I placed a magnet near there to hold the string back against the firewall.
From this I derived that the hole for the fan motor is 3&1/4 diameter with its center being 9 inches from the center of my string reference hole at right.
Center of 3&1/4 hole is 1 inch below the string line.
Whatever else you need can be added.
Hope this helps