i have yet to actually see anything with dual wheels offroad so no comment on how they actually handle out there

 

Dual wheels of road are great.. I use log skidders that have dual wheels and log trucks that have dual wheels, they also do great, better than those new big trucks with single wide tires in the rear. The skidders have more flotation so the stay on top better in soft stuff.
As for dually conv, you will need the spring perches off the doner truck, these set the springs at the correct width for the dually rear end.

 

Ehh.. Dually pickup's do ok in muddy situations, but no better than your average truck or SUV. Trail wise is a fail for a dually, the wheels stick out further and can make it real tough to squeeze through palces. For use on a farm, the only real difference I found personally.. was the dually seemed to spin the rear axle much more unless a good bit of weight was present.
IMO, you would be wasting your time doing this conversion on a K5 for the reasons your looking at.

EDIT: Here is a Semi-truck rear and front end Dually K5:

http://www.youtube.com/watch?feature...&v=wtIo6dGI47A