I just did this a couple months ago. I used 3M weather-strip adhesive; the yellow, gooey stuff. I pondered a very long time what to use, I thought silicone would take too long-I didn't want to brace it or worse, hold it in place for 24 hours. Spray glue is not forgiving if one needs to shift it slightly.
With the 3M stuff, I followed the directions: Coat both surfaces completely, let dry. Just before installation coat the weather-strip again and go! (Our "weather-strip" is the dash pad).
Because the piece is so big, this is the part where you gotta move fast!
(This is also the part where you're in a panic any way because you never got to practice this before, sure you've done your fair share of engines, ball joints rear drums, tune-ups, SU adjustment all that stuff that can or could be done again or over if it ain't quite right, you spent the last few nights test fitting and trimming the edges and test fittingandfrettingwhatifthiscomesoutlikethelittleprojectididlastyearohwellicanalwaysgobackandadjustthatifi'mevernotsolazybutgeewhizglueingadashpadallpermanentlikehowdoyougoaboutlearningorpracticinghowtodothat?).
I'm sorry. What I mean to say is it could be real easy to use a semi-fast drying glue if you know exactly how it's going to roll onto the dash. You'll know this in your sleep after all the test fitting and trimming.
Have fun!