What is the diff between the Extreme Duty kit pn 12370835 and the production LT4 timing set which uses

12555885 - Camshaft sprocket, LT4, true roller design
12555887 - timing chain
12555886 - crank sprocket

i saved about $75 but getting the 3 parts by them selves...i didn't get the gear for the waterpump drive, but mine wasnt worn anyways.

 

The LT4 Extreme Duty uses billet steel gears rather than the PM gears, and the Extreme Duty chain is a heavier single roller.

 

http://www.grandsportregistry.com/lt1vslt4.htm
Nice chart to show difference

 

That page compares LT1 stock components with LT4 stock components. There's nothing there about the HD timing set.