2002 cheyv 6000 engine misfires on # 1 and 3

Would be glad to post a list of the possibilities - however that isn't really going to help .. This will require *hands-on* testing with a professional quality scan tool to determine exactly what is causing the misfire. Cylinders 1&3 are side by side - can look for a vacuum leak that is related - around the left front of intake manifold - beyond that I highly recommend seeking professional help since you've already determined the ignition system is ok (?) .... Injector balance test along with a compression test and a 'FULL' scan test etc., will be required. Good luck with it!