The entire body will be raised up about 3" unless you raise the rear floor of the cab up to accommodate the different frame. I have done this about 28 years ago for someone else. I will never do it again. It is a lot of work for no real gain over just keeping your original frame and upgrading the suspension, steering and brakes using '73-'87 components. Here is a thread about it.
http://www.67-72chevytrucks.com/vboa...d.php?t=481023