Agree with Colin ,
Rears , use the 5 leaf springs ,IIRC the 4 leaf are not really designed for the heavier +8 , more suitable for the 4/4 and +4.
Fronts,
Again follow Colin's advice and talk with Peter Mulberry. Do not use anything less than 130, 140 is about right unless you use the Suplex system with rate risers.
If you are rebuilding the suspension don't forget that the right dampers are as important as the right springs.
