If I remember, both the stock spring from my '99 were the same.  I know on
1500's, the driver's side was rated higher, but the HD trucks used the same
left-to-right.

I think it will give him some lift, since the stiffer spring won't compress
as much.  However, if I remember, this guy already has 2" coil spacers, and
is looking to add more lift with the springs.  That's going to negatively
(no pun intended) impact his caster angle, probably beyond what the
adjusters can deal with.  Most 3" lift kits come with longer lower link arms
to offset this.