Windshield gasket replacment

You wont like my reply because it will cost more. Front windows get sand blasted and cloudy. One would never realize just how bad the old glass looks until a new glass is installed. My advise comes from my own experience. Try to carefully remove the glass without damaging it or the gasket.
A bunch of us have had or read of bad experiences related to the fit of reproduction gaskets. We choose to clean and reuse the OEM gaskets. This means the gasket you remove can be sold. There's a market for it.
If/when you crack that original glass there goes more money. It really is a blessing in disguise though.
Looking at the new glass from inside or out makes the end result well worth the effort.
Good luck
As for where your water leak is... Unless you really dig in and find it, you're only guessing the window gasket is the cause. For example, Some vinyl topped cars have a small piece of trim at the top of the a pillar post. That piece of trim has a screw in it.
If the metal is rusted around that screw, water can stream down the inside of that post.