I have installed one in a 68 Charger without removing it. I had all different types of weights, wedges, etc... on the dash until it dried and still had one little spot to come loose. However I was able to fix it but didn't like the idea of it coming loose.
The second one which was my 67 Barracuda I removed the dash pad from the dash. I clamped the cover to the pad with those big plastic clamps (I had a bunch of them) and it did a excellent job. I clamped it from the back side in which I couldn't have did this in the car. Left it like that for a day or so then installed the dash. I'm just saying I had much better outcome on the second one. Some of us are limber and flexible so reaching the bolts isn't a problem I guess. It actually wasn't that bad to remove it.