Yes, common rust areas. You will need to purchase new cab mounts and floor pans or make your own. (easier to purchase for most of us.) If the rust is that invasive, you will want to look at the inner/outer rockers, lower a-pillers and cab corners also. They will need to be welded in and check out about 50% of the build threads for different methods of installation.
Other common areas: hood: front, side edges and inner supports, roof lip, air vent pocket bottoms, front/rear or front fenders, grill supports, radiator supports, and the beds very. Most common issues I see with both fleetside and stepside is front bed panels, rear cross sills and the tailgates always take a beating.
It'd do you some good to look in the FAQ's section...will answer a lot of your questions and is a great resource to help any member educate themselves on these trucks and many great write ups on different projects and upgrades.
Link: http://67-72chevytrucks.com/vboard/s...d.php?t=559678