Isn't he going to be hit with having far too much lift in the rear? Airbags are really your best bet. Firestone can fit you up with some.
Shouldn't, Chevy springs are only good for about 2-4"of lift(most I know only get 2), the combination shouldn't be much more than the lift yield from the Chevy's and the thickness of the Toyota springs. How much lift the combo would have can't be known until he builds the rig, and loads it with the base amount of gear he will carry all the time. For my '90, I run one additional main and one between the 1st and second Toyota leaves, the additional leaves level the truck out, carry the daily load, and the additional 500lbs of gear when wheeling.
I think the ride quality of the Ag tires will be the determining factor, if they can't generate a soft ride at low psi, the airbags would be a better option.
