I used Eaton Springs (cheaper through NPD than Eaton) (how does that work??)

I purchased the 'install kit' for front and back. I rebuilt my entire front suspension at the same time I did the front springs. The install kits come with every stinking bolt and nut that you need to do the install, so well worth replacing everything at once. Spring rates go by engine size. Rear leafs for 400 coupes were 4 leaf, not 5 leaf as one might think.

I agree with the other comments above as well. You just have to decide how far to take it. But its easier to do it all at once rather than over the course of multiple repairs.