If you have a welder just weld in some blanking plates of sheet steel. I used what I had on hand - rivets, epoxy, and aluminum 1/8th inch thick sheet.
Lucas toggle switches had barely enough depth in the mounting thread to clear the thickness of my blanking plates. Plain sheet steel would not have been a problem.
Fiberglass impregnated 3M bondo worked great at filling gaps, seams, leveling out the dash panel and building up the tapered top for a smooth surface. The vinyl will show every defect underneath - take your time and treat it like prepping a body panel for paint. Lots of sanding & prep.
Contract cement works great - unless - you "f" something up and have to pull it back and reposition it - you will end up with a "scar" of glue underneath.