AHHHHHHHHHHH!!!!!!!
NO SILICONE!!! NEVER EVER EVER EVER!!!!!!!!
Silicone is the worst possible choice for any non-engine related automotive application. It will not only trap water, it attracts it, which means big honking rust problems in short order.
A windshield install is not as simple as it used to be. When I was working my way through school, I was an autoglass tech for several years. Back then (late 80s, early 90's), a butyl install would have been the method of choice for a 914, and was similar to the factory seal. Now, there is some question about whether it is a good choice.
The reason is that new urethane compounds provide a dramatically stronger bond to the frame, and thus more structural rigidity in a roll over. No question that is a good thing. Modern standards require a minimum retained bond of 70% after a roll over. Urethane can do that, butyl ain't even close. Unfortunately, 5-10% is a comon average for a butyl seal after a roll over. So, if urethane is so superior, why the question? Well, because modern cars rely on the structure of the windshield bond to protect the occupants in a roll over, but 30 year old sports car don't. Nobody in 1970 Germany considered the windshield frame a structural issue for a second. The sealants used at that time simply couldn't do that. So, we have a built in roll bar in the targa bar instead.
Sooooo, the question is, do you really need the added strenght of a urethane seal? I used to say "no", but after a long talk with by buds at the glass shop, I have changed my mind. Seems to me, given that there is no down side to urethane, it is a no brainer. Unfortunately, as I learned checking up with my glass shop buddies just last week, the modern urethanes are probably not a DIY proposition. You will only be able to get the right stuff at an autoglass shop, and application is not as straight forward as popping a tube into a caulk gun and lettin' er rip.
If you think must do the install yourself, I would reconsider. Having a shop come to your location and doing it will not cost that much ($100ish), and is the better choice. On the other hand, the chances that a butyl install would ever bite you in the ass are very slim. For me, often carting the offspring around in the vroom vroom, that is a chance I'd rather not take.