Screwing into the headliner

Make sure you hit a crossmember or it is going through the roof. When you drill the holes, get "drill stops" that clamp on your drill bit with a set screw so when you bust through you don't go through the roof. Use sheet metal screw with a blunt end (usually come in brass), so that if you screw them in too far, you have less chance of making a point in your roof. Most of the roof area has a single skin of metal. You can see where the crossmembers are, but you should really consult a set of drawings from someplace like AllData so that you don't drill through a wiring harness.

Will do, gotta look for a set of plans. Thanks for your help.

Yeah, but I heard once you take the headliner down it'll never be the same again. Plus, it's a pain to take it down.

You can always replace it with shag carpeting and call it Retro-plorer.