Can anyone tell me what material is used to fill the gap between theI can't speak specifically to the '75, but on the '71, the seams were all covered with tape (it looks pretty similar to duct tape), and then a piece of padding (probably about 1/4 inch thick) was placed over the entire rear section of the roof. No actual filler was used at all around the spacer insert.
fibreglass LeBaron rear window insert and the metal on a '74-'75 Imperial?