If you are going with an original style headliner (two piece front/rear) there is no rubber seal that goes specifically with the headliner. As already mentioned, it tucks behind the top of the windshield rubber and the door windlace. The back is secured with a curved brace and 11 sheet metal screws. For the center (between the two pieces of the headliner) you would need a floating bar. That also is supported behind the windlace of both doors. I believe the floating middle bar is available through some vendors.

I do not know much about the one piece plastic version that is being sold other than some folks swear by it as easier to install. I was going for an original/simple look so I installed the cardboard original.

Hope this helps.

Steve
55 1st Series 3/4 ton