when installing an underdrive pulley, do i need to replace the belts or can i use the stock belts?

 

You have to get new ones.

 

is there a special kind i need to get or can i just go to a locla kragen or autozone and ask for under drive pulley belts?

 

Whoever sold you your UDP should know. Either that, or find out who made the UDP and call them. There's no such thing as a 'UDP' belt, you basically have to buy a smaller sized belt that is the correct sized based on the reduced diameter of your UDP.

 

You need 2 replacement belts:

These part #'s are for Gates brand belts:
K-060408 & K-040292 or K040300

You should also replace the oil crank seal when you install the UDP.
Nissan oem part #: 13510-31U00