Might be different on a '92, but on my '84, there was about 1/2" of clearance IIRC. That space was filled in with insulation padding.
There's a lip around the edge, that in the front and the back extends about 3 or 4 inches. At this point, the headliner sits flush against the lip. If you'd like, I can take some pictures of what my setup looks like...I pulled the sagging headliner in 2003, and still haven't put it back in...either going to re-line the current headliner or replace it; that's still up in the air, and I'm also going to renew the insulation when time and $$ allow...