do a droped crossmember, it's much easier whe you think about it. if you z the frame you have to worry about motor to cab clearance issues along with pinion angles change, exhaust routing, all that sh*t, it you crossmember notch it all you need to worry about is maybe some steering column clearance to the upper arms. And no it doesn't mess with the geometry, it just moves the whole front suspension up. check out this link it'll tell you all you need to know.
http://67-72chevytrucks.com/vboard/showthread.php?t=60649