Early A windshield replacement. How hard???

Yes you need to seal the rubber gasket to the pinchweld pocket on the car and I like to seal the glass to the rubber gasket too, just to be sure.
I use an original style sealer called butyl rubber windshield sealer. (do not use silicone sealant it will cloud the laminate in the front windshield)
You can get it from a glass shop or I sell it on ebay.

Here is a link to some instructions for re-sealing the glass and installing the lockstrip, along with the factory instuctions.

http://home.comcast.net/~mazmatic/instructions.mht