Just a driver doing some long term repair on a truck

Good morning guys, figured it’s time to join a forum about trucks, hopefully you guys might have some answers for some questions I have along the way

 

First question

took the roof off due to the typical to many holes, to large of holes, to much junk on my roof. Replacing with a fiberglass unit from Home I’m not positive on what I should use between the two misting surfaces of the fiberglass cap and the aluminum. It appears to have had Butyl (the same as glass on cars) but curious if you guy have any other ideas non silicone based stuff

 

As she was when I got behind the wheel, to just before tear down for repaint and such