For a 5th gen 6.2L I think they only offer the A/C pulley in stock size. The main shaft is under drive but the A/C pulley is OE.
Thats how mine is anyways.
I have the 10% UD with OE A/C pulley. P/N 918629 sub note "1"
The belt sizes are listed at their web site by Napa P/Ns under the sub notes.
Mine is 25060827 for the main belt and 25040353 for the A/C
http://www.atiracing.com/products/da...serp.htm#Zbody