Left for dead: 72 2wd blazer build

[nonamecharlie]

My truck is starting to leak in this area, after looking at your picutres I am convinces it is coming form this place under the cowel. The problem is I am not sure how to fix it. I want to stop the leak to save the door jams and the side wall before it gets bad, but I don't want to disasemble the entire sidewall like this.

Is there a rubber seal I can replace to stop the damage? I don't have much damage right now, I think just stopping the water from getting in will be enough.