Shawn - Carl - Justin:
I have a '69 Camaro that was a SB, now a BB. The heater box is for the SB. I am in the process of converting it to a BB heater box system. The SB heater box has the inlet-outlet tubes exiting the firewall from the right side of the heater box (looking at the heater box from the front of the car). The BB heater box has the inlet-outlet tubes exiting from near the center of the heater box cover. This is so the tubes will have clearance with the right side valve cover and right side of block / exhaust on the BB.
Anyway, there are more than 4 screws to remove, and that is why the darn inner fender skirt or, really, the fender needs to be removed. There are 2 screws up under the fender that go into the flange around the blower motor. (The total number of screws is, I believe, 7.) So, there is no way to get the heater box off without removing the inner fender skirt or the fender.
A mechanic friend told me that what "they" often do (or did) when they had to replace the heater core was to unbolt the inner fender skirt and pull it down and out of the way. "They" would wedge it out of the way with a short piece of 2x4. I also read about this trick in an article on this problem.
However, as I struggle with getting to my heater box, I am having zero luck at moving the inner fender skirt... anywhere! I do believe I am going to have to take my fender off unless some miracle happens soon.
It doesn't surprise me that the former owner of your car cut a hole in the inner fender skirt!! He must have been very frustrated. This is a job for PATIENCE!
Good luck. Post your findings if you have time. Thanks.