bench racing sort a .... if you had a choise of springs(40 or 60) to use on a 40 with 60 axles what would you use and why .... flipped for sure.... but rear 60 just 40??? I know 55 rear rock but lets leave those out for now...
60 rears are a nearly centered pin, flipping won't net ya much.
I'd start with all the leaves tho, easy to drop one-two if it's too stiff.
Remember the 60 packs have larger spring eyes, and are pretty tight in the 40 solid-side hangers...not much issue if you do custom front mounts to outboard the packs a bit, but might be an issue in the rear.