hi schneiderman,
i've had my dash out of my 68 barracuda several times and i think it's the same basic assembly that's in your 70 duster. here's what i've learned:
1. if you are not an overweight old guy (like me), you can change bulbs and some other stuff without taking the dash out. i've done "non-dash removal" work on my cuda by getting a very bright trouble light and putting it on the floor of the pasenger side and then laying on the drivers floor face up using one of those "mirror on a stick" expandable mirror tools you can get at most parts stores - look all around the back of the dash and with a very bright light and one of those mirrors, you can get to pretty much everything on the back of the dash.
2. removing the dash. that makes everything easier. if the steering column will not drop down enough even after you've removed the two bolts holding the column to the dash, you could try taking off the three bolts on the column itself on the firewall under the hood. these columns bolt to the fire wall with three bolts and if you losen these, you should be able to drop the column down.
now, as to the dash. obviously, take out every screw that you can see on the front of the bezel. there will be screws on the bottom of the face and there should be a couple of screws that are on the top of the bezel that you have to bend over to see - they go vertically into the top of the dash. all these screws have to be completely removed - it will not work if they are just lose.
the cigarette lighter, windshield wiper and light switches all go thru this dash and have collar nuts on the back. i would suggest you remove all these switches. the cigarette lighter is easily removable by unscrewing the round assembly from the back of the heating element. for the wiper, pull the knob off, then take two small flat screw drivers and unscrew the recessed collar nut on the front of the switch. this nut is difficult to get off if you do not have the proper removal tool (you can buy them on ebay if you want). the light switch comes out by: feeling under the switch for a small "button" - push that button against the switch housing - the light knob should then pull out of the front of the dash. once the knob is removed, unscrew the collar nut like the wiper nut.
as a final point, you may also have to remove the radio as there are collar nuts on the volumn and tune controls and a third support bracket/screw on the back of the radio. if after removing all the noted items, the dash still seems "attached" at some point, craw under the dash and shine a light where the attach point seems to be and determine if there is another screw.
once all the attaching screws are out, your dash should be very lose. if it isn't there is still a screw somewhere. remember, this plastic is 45 years old and is very brittle. don't stress or pry much on this dash - it will break.
good luck
