couple things here, those using cheaper glass not OEM found the contour and shape of the glass different and difficult to get to line up and for trim to fit properly - not me but i am telling you what others have posted i verified before replacing mine.
2. the clips- yes first clips from 914rubber had brittleness issues, not the newer more recent ones.
also , i got my glass from the Porsche dealer parts, $400 i believe for the windshield and only $100 for installation , so check with the parts department at the dealer, I know mine well and they use a independent guy that does all their installation, both new and old cars, so he was experienced in both and even picked up the windshield from Porsche for me and came to my house! i had it ready to go - remember prime the surface , apply the butly tape, i let them place the window and i put in the trim afterward. aftermarket trim has a reputation for not fitting as well as OEM too.
And, clips have to go in before the window.
other than that take your time, with the trim its difficult, if you hve nice paint or new paint i would tape off the area its easy to slip and scratch your new paint job.
there are a couple jobs in reassebly of my car i can do with out ever having to do again, the most frustrating was installing the triangle glass and seal, the other the front window trim, id rather pull and engine and transmission.