The instructions in the pre-54 Shop Manual call for sealant between the rubber and the cab. On my 54 windshield I also sealed where the glass goes into the seal (in that year, this is called-for). The windshield does not leak now - it leaked significantly before I took it out and added the sealant.

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0104.htm

I got this sealant at NAPA and at another local parts store.
3M Sealant, Bedding & Glazing Compound - caulk-gun tube

That bedding/sealant/glazing compound easily cleans up with thinner, turpentine, etc