[SS427]

Assuming you are talking a 68-72 A-body or your Buick, all you have are the two bolts left, leave them! The two bolts at the blower motor end will retain the engine side housing in its original position and have nothing to do with the inside box. You only need to remove the 5 nuts on the firewall side. That being said, it can be a little interesting to get the inside box studs and the heater core hose nipples to be reinstalled into the housing when you are done but I have done it. You also do not have to remove the control cables. Simply drop the box and "roll" it on its side so that you can remove the core. I believe your Buick has plastic inner fenders so you may actually be able to twist and remove them but I would not recommend it. If your car has is an AC car (obviously your Buick is not) that is a whole different monster.

If you do feel you need to remove the last two bolts, all you have to do is drop the inner fender down by removing all the attaching bolts as well as the battery tray to inner fender bolts and pull it down enough to get your arm in. Protect the frame with towels between the frame and the inner remaining bolts. I find it easier to remove the blower motor if you are doing that to take some of the weight off but depending on how far you drop the inner fender down you may not be able to remove the motor. I have done this a few times without any damage to the car or the inner fender.