Long-time reader, first-time poster (poser?

)
As a long time owner of a '99 Buick Regal LS I can tell you Chairboy that you have a 1.5 din opening there. (My original plan was to install a system in the Regal; sold it and bought a Tacoma.)
It would be difficult to use the existing dash opening to shove a lilliput screen in there flush. You best bet is to follow King_Aero's comment (check out DigitalWW) or fabricate some sort of mount of your own. There's a useless ashtray compartment in front of the shifter. You could always find a way to take advantage of it.
Good luck!