Does anyone know what kind of sealent to use between the chrome channel (which holds the door and window whetherstrip)and the underside of the roof on a 68 Chevelle?

I believe the factory had some grey colored rope sealer in there. You can use just about anything you want because it will be covered and it has little chance of leaking. I used windshield ribbon "butyl tape". Just stretch it till it's thin and use it like a gasket.
Roger

I uses buytal windshield caulk.
Got it at NAPA in a tube for a calking gun.
Also, get some weatherstripping cement for the ends of the weatherstripping. Also used in the trunk and the door if you want.

Thanks