I had my dashpad off already, so it was nothing at all to do it. If you have the OEM brackets, they attach with one or two screws each and it is done in just a few seconds. I guess reaching under the dash to route the wires took a few minutes but, again, not that hard. As I recall, getting the dash pad off, it took maybe 5 minutes to locate all the screws and pull it off. So, in short, not hard at all.
Here's a thread with some photos by somone else.
http://67-72chevytrucks.com/vboard/s...d.php?t=344303