I have done a search and read the posts about removing the heater core. I have a '68 AC car. I am having trouble getting the heater box to come loose from the inside of the car. It appears that there is one stud at the bottom right side (looking foward from the passenger seat) of the heater box that will not come loose. I have tapped on the stud from the engine side since my fender is off, but it will not move. There are two other studs on the opposite side that have released and I am able to pull the left side of the box out from the firewall from inside the car. Does anyone have any ideas on what could be holding up the bottom right stud? Thanks

If the nut is off, probably the glue between the firewall and heater box holding it on.

I know on non-AC heaters, as mine is, there are six out of seven nuts to remove. The seventh holds on the blower plate on the engine side of firewall.

Could it be the kick panel in the way?

Thanks for the help Everett. I discovered the problem. I removed the cover on the engine side of the fire wall and there was a second nut bolted directly to the firewall that would not let the stud slide out from inside the car. Is that normal? I was under the impression that I could remove the heater box from inside the car without having to remove the cover on the engine side of the firewall.