I'm in the process of bagging a 68 c-30. First a few questions:
Are you planning on keeping it 3/4 ton, or would you convert it to 1/2 ton?
Do you want everything to be bolt in or can you weld?
Do you want to lay frame, or just bag it?
The easiest thing to do would be to convert the truck to 1/2 ton and use the newer style 5 lug hubs.
Air Ride technologies recently released a rear kit for those trucks that comes with trailing arms setup with bags and c-notch brackets for the frame. That is complete bolt in and would get you pretty low, but not frame laying low.
Pair that with a set of control arms and some good bags, and you will be plenty low, just not laying frame.