i am lookin at buying a set of underdrive pullys but i dont know what size belt that it uses or if it uses the stock size

 

i can tell you its not the stock size and it varies on the pully/set that you get. the instructions should tell you what size belt to use

 

I have a 5.9 with march underdrive pulleys so it's different.(alternator and crank pulley only) The instructions said 1" shorter. I couldn't find 1" shorter but I found .5" shorter and 1 7/8" shorter and the 1 7/8" worked with plenty of room for the tensioner to move so it went on easy. I keep the part number in the glove box for later replacements as I looked all over town and only found one in a warehouse that worked. Bigger city would probably make it easier.