I’ve got a new front windshield molding for my ‘87 GT and have cleaned out the old sealant in the track. My question is , what is the best adhesive to use to hold the new one in place ?

I used DOW Betaseal U-418 from the local parts store. You can also use it to glue the rubber molding in place; it may otherwise not want to stay put in the corners.

I started the job with two tubes, to guarantee that I wouldn't run out mid-job. I ended up using 1-1/2 IIRC.

I reused the plastic spacers that were originally there.

Now is the time to clean up any rust, if applicable.