I actually did it on my 67 Camaro, and it was like night and day. I'm not sure how well it sticks to the roof, if you removed the headliner, but I'm planning on doing the same when I get the car painted. I used a heat gun to make it stick and mold even better.

There are other brands out there that are cheaper than Dynamat. Ebay has a few vendors. I think I might go with fatmat.