Apparently not, I haven't got my built engine running yet but the posts on NS.com from the people who fitted these said that they needed to buy new belts. Still they are only around $15 each, and i'm not running an aircon so i'm only up for $30. Compared to the $6000 i've spent on the engine i think i can lash out on some new belts.
EDIT!!
Found the post from NS.com;
Some numbers:
Stock ---- PowerBond 20% underdrive
Weight grams: 3285.1 ---- 2554.4
Belts:
Alternator 4PK0855 ---- 4PK825 approx or nearest
Aircon 4PK0845 ---- 4PK815 approx or nearest
PS 4/3PK0835 ---- 4/3PK800 approx or nearest (check if yours is 3 or 4 lines)
Diameter ~150mm ---- ~120mm