From the threads I just found on the subject, the opposite is true, which is news to me: if it's a DMCH A-pillar then it will be plastic, otherwise it will be fiberglass -- I thought DMCH was selling fiberglass from the time they ran out of NOS?! Hopefully someone can clear this up, but it's also dated info. OEM was plastic, but I can't fathom DMCH actually sold any reproduction plastic ones. Regardless, if you're replacing it, fiberglass is a must here, plastic just doesn't age well and before long won't hold up to the heat like it used to, and you'll replace it yet again. Fiberglass is about the same price anyhow, why not do it right the first time?
Here's the threads I found on the subject where a vendor weighed in, hope they help:
http://dmctalk.org/showthread.php?11...a-pillar-trims
http://dmctalk.org/showthread.php?36...-A-pillar-trim
One additional one on the windshield:
http://dmctalk.org/showthread.php?60...ld-replacement
If your windshield is removed, you need a jig. I've seen the results of not having said jig, the end product is roof sag. Doors closed still has this problem, the windshield is structural. It's not hard to make, actually a regular car jack and some lumbar is sufficient, but it will avoid any warping issues and also allow you to safely and comfortably get in and out of the car without crawling in through where the windshield once was. It's not a terribly noticeable sag until things start not fitting like they used to, and with the natural heat/cool cycles of just being outdoors, over time you'll notice it.