Yeah don't worry about doing the engine compartment. What you want to do is remove all the carpeting, and get as high up as you can up the firewall with just a single layer of deadener. What's really going to make a difference in blocking out engine noise from the cab is closed cell foam such as Ensolite.
After deadening the inside of the firewall as much as you can, use the spray adhesive and line it with a single layer of ensoilte foam.
You can get ensolite foam cheap from Rick at
www.raamaudio.com