That's exactly how I did my wife's dash. Expanding foam in the cracks, the sanded everything down flush. While you're at it, you can add an extra screw or two to help hold the defroster ducts. I also welded the screws to the pad piece. They are phillips screws that come down from the pad side and bolt it to the front dash piece. JB weld would work if you don't have a welder. You'll understand what I'm talking about when you actually take the pad off.....or try to
Just make sure you get spray glue that is heat rated. Either get it at an upholstery shop, of some of the fabric stores around here carry it. I haven't seen any with a heat rating at either Lowe's or Home Depot.....so don't get their stuff unless you want it to release when it's out in the sun. If it doesn't specifically say it can be used for headliners, don't get it.