A few pics of my 1999 K2500 with 1.75" lug centric(aluminum)spacers on the rear, semi-floating axle, tires are LT265/75R16.
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They have hauled and/or towed quite a bit over the years.
In 2005 when I installed em, I couldn't seem to find steel ones, only the aluminum.
If I had to replace mine now, I would probably go with steel.
From this thread here:
http://www.gmt400.com/threads/8-lug-wheel-spacers.25542/
A link for steel spacers :
http://www.wheeladapter.com/single_spacers.php
Not saing you need spacers though, I think that 3500 looks good now.
But if you have the dough, it probably wouldn't hurt to try em out and see.
Best of luck.