The inside of a 50' door looks like this.
https://www.flickr.com/photos/advance-design_parts_co/14184302391/in/set-72157631868452754
As you can see front and rear channels mirror each other, front is welded in, the rear attaches with two screws on the bottom and one on the top. The way you are describing it sounds like the old channel is still in place. The front and rear channel guides are formed sheet they have nothing inside them.
This is what the channel looks like once it is removed (they are right/left hand specific).
https://www.flickr.com/photos/advance-design_parts_co/9479860265/in/set-72157631868452754
Here is a slide set that should also help. I think you still have remnants of the old weatherstrip in the lower door, it all has to come out for successful replacement.
https://www.flickr.com/photos/advance-design_parts_co/sets/72157641244124604/

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr