Need help removing somethings

You gain valuable access to much of what you need to remove under the dash.
Your '74 Duster won't have the "J" hook or fresh air door that fstfish66 mentioned. Right behind the glove box, it does have a bent-metal support strap on top of the heater housing and attached under the dash to a stud.
You also have to get up under the dash to remove the flex hoses running up to the defrost vents.

Before you disconnect and drop the heater housing, remove all of the vacuum lines and control cable (if equipped). I assume you also have the air diffuser. That's the piece hanging under the lower edge of the dash that you bust your knee on. Remove it, too.

Move on to the engine bay side and disconnect the wiring harness to the blower fan. Remove the rubber vent hose and three nuts holding the fan and pull it out. Disconnect the two heater hoses at the firewall (probably should drain coolant first). Remove the plate that surrounds the heater core copper tubes. Remove the remaining nuts securing the heater housing.

Go back under the dash and pull the housing straight back so the heater core tubing and mounting studs all clear the firewall. The tricky part is trying to find the right downward and sweeping motion that lets the housing drop down into the passenger floor. I'm not sure about a non-A/C car, but do you have a rubber drain hose coming through the firewall right about where the brake line bends downward on the passenger side?

That should get you started. I probably forgot something, but most likely you will be able to figure it out. If not, give us a shout.