I think it might depend on which 1999 motor you have. Is it a L37 or a LD8? The L37 motors have a higher compression ratio than the LD8s. I'm not sure if that comes from a different piston or a different shaped combustion chamber in the head. If it's a different piston there could be valve clearance issues if the max lift on the 300HP cam is higher. Compare the two cams side by side and measure the lobe height. If the total lift is about the same and the only difference is duration than they'll probably "fit". If I had to guess the extra horse power comes from more max. lift on the cam.