You don't need to bodydrop these trucks to lay body. If you do a notch and link in the back it'll lay no prob. up front, I suggest sectioning the crossmember, this, contrary what people say, is easier than z'ing the frame. When you z the frame you have to worry about all kinds of shyte, sectioning the member is fairly simple and if you do it right there are only two things you may run into problems with really, stearing shaft to upper control arm rubbing, and tie rod rubbing. This will lay the frame flat front to back, but if you still wanna lay body... These are the numbers from my truck, the frame is only 1/4" lower than the pinch of the rocker, and 3/4" if you cut the pinch off, soo theoretically to lay rocker you need to bodydrop 3/4", but who wants to bodydrop that little when you can be creative and get that 3/4" out of the fat OEM body mounts, think about it....I actually just did the measuring last night and gots me a crossmember to hack up today, $10 is a steal.... Here are a couple links to check out and you'll have all the info you need.
http://67-72chevytrucks.com/vboard/showthread.php?t=60649
http://www.captkaoscustoms.com/chopcross.html