When u say the plugs were heavily corroded from fuel that worries me because a excessively rich condition would not cause the plugs to corrode ..were they black n carbon coated like a black looking cruddy on them or were they a white grayish color coated n looking they were living ok n the ocean floor??..if you had the typical fuel line leakage that they all get then if anything you would be running leaner than normal from a droo on fuel volume...do you have a scan tool n van pull some o2 sensor data ?? Any other codes besides misfire on 2 And 4 ... how ...there's a enormous amount of possibilities that can cause your situation I. Without any type of data from o2 sensor MAF TPS knock etcetera we would be just doing the old shotgun theory of throwing up a bunch of random guess n hoping I be might be right n that's defiantly not what h want to do because it always ends badly with either rod knocks burned ringlands.heafgaskrts 99% of the time before u guess what actually was wrong....valve adjustment?? Leaking heaxgaskets in 2 and 4 .intake gaskss leaks cuasjng low vacuum n return chasing a rich condition. Stuck pcv valve n on m on n son non n on..