Need help !!! CAR:97 GSR

I have a engine check light on it reads codes 72 and 74. I just recently installed a Greddy turbo kit. I replaced the sparkplugs with new one's. Also when i drive its runs smooth no misfire or smoke in the back. What could be cause of this or someone has installed the a Greddy kit with the same problem.

thanks

 

72 random misfire cylinder 2
74 random misfire cylinder 4

maybe you can not hear the misfires? check your timing