On a 74 can you just pull the front of the heater box and put in a new core..

[ramairthree]

or does the whole box have to come out?

I searched and it seems it can be done without pulling the dash.
Maybe at least take out the glove box though.

Thank you.

[Back N Black]

You don't have to pull the dash, but you will have to pull the entire heater box.

[E5 Charger]

You don't need to remove the whole box to just do the heater core on a 74. You would if you were changing the blower motor. Looking at the attached diagram, when you remove the Rear Housing you will have access to the core. Don't forget to install some new rope caulk around the housing when you put it back on. This will help keep the antifreeze off your carpet if you get another leak. Can you do me a favour? I'm restoring my heater box right now and the core has been out for years. I have a couple screws that are longer than the others. Can you note where they are on yours, if you have to remove them.

[ODZKing]

I would have to agree with BnB, you'll have to pull the box to get at all the screws and clips. there is no way you'll get the top clips with it in the car.
The motor comes out from the firewall in the engine compartment.
Here are a couple shots of mine after I got it back from Richie "The HeaterBox Guy".

[E5 Charger]

I should have mentioned my box is a non-air Stratovent box. It looks totally different than yours. The rear housing was only held with screws, no clips. At least there wasn't clips when I did it and no signs of any being there on the housing. Should all heater boxes have clips, or were some just held with screws? Did yours also have screws under the clips. I removed my leaking core years ago before I started the restoration. Recently I started restoring the box which was still on the firewall, the rear housing was laying on the floor with the heater core laying in it. ramairthree which heater box do you have? If there are clips I'm sure you would have to remove the box so you don't damage it trying to remove the clips.

[ODZKing]

Yes, some do not have clips.  Just keeps the sections secure.

[gtx6970]

You can probably do it with the box in the car,,,but it'll be much easier with it on the bench