You are going to want a Factory service manual. not a cheap copy of a assembly manual.
Here is one on eBay as an example.
http://www.ebay.com/itm/1970-PONTIA...578256?hash=item33b0534110:g:8Q4AAOSwX~dWltTb
Removing the dash from your Pontiac is not that difficult however I warn you this is not a job you want to tackle on your own.
You will need help removing and reinstalling the bulky dash and reinstalling the steering column.
You will need to be prepared financially to replace anything you may break while doing this job.
I certainly would not attempt this job without having a new wiring harness in hand ready to install. ( because the age of the harness )
Take pictures many of them for wiring harness routing, vacuum line routing etc.
While the dash and column is completely together.
Label each and every connector you can while the car is still assembled
( makes sure to label each side of each connector )
When you start to disassemble the dash and or center console if equipped. bag and tag the bolts, nuts, screws, brackets etc in small batches.
Do not put too much in any one bag this makes it easier to remember what goes where.
Write down the steps of what your doing and link each step to a baggie of fasteners making notes of any fasteners you find missing and high light those missing fasteners so you can order or obtain new ones.
For example only
Step one disconnect battery negative cable first
Step two remove lower drivers side dash cover 8 screws required 2 missing bag #1 take a picture of the fasteners with your phone or camera and label the picture to match your bag number
Step three remove steering column 6 bolts inside 2 under the hood bag 2 none missing
Step four Remove front seats 8 bolts required none missing bag 3
As you dig deeper you will uncover new wiring connections a vacuum lines cables etc that need to be labeled and photographed as you go.
Your dash will need to come out of the car with certain parts of the factory harness still attached to the dash.
Knowing where to disconnect the harness is key to not killing yourself or your friends while trying to remove the dash.
Having a factory manual, reading the section before you start will give you a better understanding of what is involved.
I have been forced in the past to remove many dash boards with no manual and no experience on that certain cars, however if you go blind like this be prepared to break parts and damage the dash and other components.
You will want to handle the dash as little and carefully as possible. to do this you will need to know when the dash is ready for removal.
You will get to the point where once you remove all the fasteners that hold the dash in place the dash will want to fall out of place.
Make sure to leave two screws in place until you know there is nothing else holding this dash from being removed and you have a friend remove one screw while you remove the other and carry the dash out of the car.
The key here is a factory service manual, No one here not me nor anyone can replace the information found in a manual.
once you have done a few dash removals on different cars you will see most are similar in procedure.
Disconnect the battery, remove both front seats, the center console if equipped, the glove box door and box.
Remove the steering column, disconnect the speedometer cable,
remove dash pad if separate from dash. remove the radio, the gauge cluster
remove all vent hoses from HVAC system to dash board.
locate harness connections that need to be removed, and disconnect them.
remove the hood release cable if equipped.
remove the HVAC control panel from the cables and vacuum lines if equipped.
Have a safe clean work space to put the dash and a clear path.
Verify nothing is connected to the dash then remove all but two of the fasteners, have a friend remove one while you remove the other then carefully test removal of the dash shining a flashlight inspecting for missed wires or cables etc.
if the dash is free carry it to the work space.
Again these steps are generalized do not go by this list go by what the manual says. and you will be fine.
If you read something in the manual and your unsure of it please feel free to ask questions.