It is a waste of money on a stock GN engine IMO. You need to machine the pedestals in the heads where the shafts bolt down to in order to provide clearance for the rocker arm travel.
Guys are making 600hp with stock longblock, upgraded $40 valvesprings, larger turbo/injectors/intercooler/downpipe and $109 fuel pump and upgraded stock style rocker shafts.
The shafts have been known to break on higher mileage cars so your best bet is upgraded HD shafts. Quite a few vendors sell them. I've broken a stock shaft myself.
-GNX7