The Pantera windshield slides inside a stretched gasket that then slides onto sheet-steel protrusions all around the windshield opening in the body. If either the glass or gasket is fractionally off-size, it will not seal 'somewhere' against high speed impacts of rainwater, and you'll get a wet floor (or passengers). And even if it does prevent water from getting inside the car, there is a small space between the gasket & the body (the 'windshield channel' or cavity) where condensation or rain can be driven under the gasket lip.
This cavity has no drains, so it will retain any moisture in there until it does what water always does: it penetrates any paint/primer and rusts. When the glass is removed, unexpected body damage is often exposed. I've seen Panteras where this cavity is so corroded, some wire connectors and electrical relays under the dash were full of rusty water and obviously did not work. Rusty floors are also a clue! Once the glass is out, the fix is to get in the windshield channel with a rotary wire brush on a drill and clean out ALL traces of rust. Then paint with several coats of a good epoxy primer & let cure overnight or longer.
You can also do one or both of the following after rust removal but before priming: carefully drill a 3/8" hole sideways and downward from both extreme lower corners of the windshield cavity into the cowl, on the other side of the front firewall. The windshield cavity will now drain any collected moisture into the cowl, which DOES have a drain pipe on each side. And/or you can apply a bead of transparent RTV all the way around the gasket's flap where it meets the body. Fill the cavity all the way around by lifting the outer gasket flap.
NOTE- you need Special Windshield RTV which is NOT what's stocked at your corner hardware store! Regular RTV cures by exuding Acetic Anhydride, which instantly reacts with atmospheric moisture to form acetic acid (vinegar). And this acid corrodes steel faster than the water you were trying to eliminate! If you smell vinegar around your RTV, do NOT use it on a windshield. It's not harmful to most humans but it corrodes nearly all metals.
Special windshield RTV cures by exuding acet-aldehyde which is an odorless gas that rapidly dissipates and causes no corrosion. It is available at professional paint stores everywhere but again- NOT in hardware stores. It is somewhat more expensive than common RTV and will be labelled FOR WINDSHIELD GLASS. Follow its directions exactly.