This is a list of Gates 6 rib serpentine belts.
http://www.gates.com/oreilly/PDFs/SizeListings.pdf
The
13/16” (20 mm) Top Width section should be the area on the chart you're looking for
If you run a string around the the routing of the pulleys (includng the tensioner) then subtract .75"- 1.0" for the tensioner.
The alternator V pulley can be swapped for 6 rib pulley. This list will give you shaft dia and offset dimensions.
6 Groove Serpentine Alternator Pulleys