It sounds counter-intuitive, but a greater number of leafs does not mean a stiffer spring or higher stance.
For '68, the 400 model came with 4 leaf springs. The 350 model came with 5 leaf springs.
For '67, the rearend had both rear shocks aligned. In '68 they changed to staggered shocks to eliminate wheel hop.
That's why '67 models came with one or two factory traction bars. (again, depending on model)
Different springs have different spring rates, regardless of the number of leafs.
I would recommend you have a telephone conversation with someone from Eaton Spring regarding what set-up you have, and what you are trying to do to it.
Regarding the hardware and pads, most everyone sells a kit to go along with their springs, once you decide on those.
I just found the contact info from the guy I talked to back in 2008: No idea if Mickey is still there 12 years later...

Thank you
Mickey
Phone 1-313-963-3839
Fax 1-313-963-7047
www.eatonsprings.com