Not hard. It won't fit the body perfectly though. Best way to "fit" them is to take a piece of 80 Grit sticky back board file sandpaper and stick it to a 2X4. Then just pull up a chair and hold the scoop up to the body. Rub the scoop's high spots on the 2X4 until it fits.
Dusty's right. The quality varies among the fiberglass manufacturers, some fit better than others but you probably won't find a set that fit well right out of the box. They're fairly easy to sand into the proper contour but you'll have to drill holes in the quarter to mount them and remove the interior panel to access from inside unless you use adhesive and bond them on.
I bolted mine on. I've never tried the adhesive although I hear it's strong and does a good job. I wanted the bolted on look, I didn't blend mine with the panel for that seamless look that some people prefer.
i know one of the companies that makes the kits has a how to on how to install it and i think they used some type of flexible adhesive and added a flex addictive to paint so it wouldn't crack in that area. (they did this with the paint on the gto's that had rubber noses) I'd just bolt them on though seems a lot easier and stronger