I posted about this around a month back, if you search you should be able to find my post. Basically for my '67 (not screw in from factory) I bought carpet tacks from home depot, 100 for a couple bucks, the head and shaft are pretty much exactly the right size (yeah,yeah ... not intended like that). I shortened a tack, drilled a hole very slightly smaller than the shaft diameter in the cowl cover after grinding down the old one flush, hammered it in, measured the height with a ruler until it was just right, then tack welded them under the dash. This only works where you have access to the backside though, in my case the car is a shell, and only the studs in the cowl cover were messed up. For any others, you can get the screw in kind as jersey joe mentioned.

Someone else also had mentioned using a stud gun to weld them on, also a pretty slick idea if you have access to a stud welder. I think they sell a cheap one at harbor freight.