Factory A/C on B-body is a total PITA....
If its factory A/C, you need to remove the
blower moter wheel found inside the box inside the car.
Your going to have to remove the lower baffle and the passenger side duct to get to it though.
If you look under the dash you'll see a square-ish
chamber to your right. Push open the door and you will see the blower motor wheel.
There is a tiny clamp which holds the wheel in place (similar to a power steering or fuel
line clamp.)
Take a pair of needle nose pliers and take the
little clamp off. Then slide the wheel forward.
The blower motor should come out afterward.
Be very careful with the blower motor wheel,
Its fins are easily broken. I don't believe there
is an aftermarket replacement for that piece.
FWIW, the factory A/C box on 67-70 B Bodies
are a total PITA. Sometimes your better off removing
the whole box and working on it on a bench than trying to fool with it under the dash.
The factory service manual outlines how to do this very nicely. Its a PITA no doubt!
If your trying to do anything to that box in the car. i.e under the front seat dash
- you have very little room to work...it gets frustrating
really fast especially if the car is a driver and
the front seats are installed.
if this is the case, your sure to get a complete
review of your church vocabulary not only for content
but for clarity!
Good luck.... get the FSM if you don't already have it !