with the door panel off, and the windows out, you drop the whole regulator assembly through the big hole in the door. The screws for the main window channel should be taken out so that you can move the window back far enough to get the vent window back it, but you do not need to assemble the regulator inside the door. I know there are a few walk throughs on how to remove and replace the regulator through the search function on the site, but this one covers the majority of the issues.
http://67-72chevytrucks.com/vboard/s...d.php?t=661177