65 Valiant cheap windshield seal

If the rubber is still soft, I wouldn't replace it. First, try "flowable silicone". You can find it at most auto stores and even hardware stores, next to other silicone RTV tubes. It is marketed mainly for sealing windshields. Pull back the rubber with a putty knife and clean well with water, then alcohol. Wait a few days to dry if needed. Squirt it all along the crevice. It seems to have worked for me on a few cars. If not, you can escalate with other sealants. One that works good on trailers is butyl rubber caulk. I think you can get it in black caulk tubes. There are also solid butyl rubber strip products. Butyl rubber stays flexible for years. I think that is the adhesive gasket they use on new cars.