I bought a Dashmat for my '71 in 1980 shortly after buying the car; it was in Las Vegas and the dash was already cracked and split. Figured the black carpet material looked better than the cracks. It came with a roll of mylar foil which you were supposed to trim to fit and put under the mat; a while later we were told not to do that because it would actually make the dash crack worse.
I had no trouble with it staying in place without adhesive but I was driving a 318 with 2.76 gears
. One thought; find some small plastic 90 degree angle pieces (I'm thinking the little twinkle-light clips with the part that actually holds the lights removed, but whatever works) and glue them to the underside of the mat around the defroster holes; with the mat in palce they extend down into the holes in the dashpad and help keep it from sliding around.
Or you could get a couple of those thin sticky silicone pads they sell for making phones 'not slip' on the dashboard and glue them to the underside of the dashmat (disclaimer! Real silicone won't melt but I have no idea if it would cause any problems for the dashpad underneath).
