1996 GMC Sierra Multi cylinder misfires

To set the senerio, for several months I have been receiving a catalytic inefficiency bank 2, have been fowling #4 and #6 sparkplugs, which I belive is due to leaking valve seals, soo, id did as chiltons suggests, found tdc #1, disassembled and replaced valve guide seals for all 8 cylinders, on the motor, adjusting valves as I went along for each cylinder, truck had actually been running fine before this other than the check engine light, 17 mpg as usuall, some slight valve noise during a load and accereration, when all was said and done, engine started right off, was quite rough and less than one minute received p0300 mulitcylinder misfire, distributor was not removed, checked that all wires were installed properly on corresponding cylinder, to no avail, replaced cap, rotor, wires and plugs same outcome, thinking valve adjustment could be wrong, dont see how really, on tdc of compression stroke, did tighten rocker nuts till pushrod stopped turning then added 3/4 turn as book says, noticed when I disassembled that adjustment seemed to be without the 3/4 turn as pushrods could be turned and rockers seemed to be loose, or right at that point of no slack, have no clue what to look into to correct this other than take it to a dealer for diagnostics, rather not go there, dont think its time really for a rebuild, was a daily driver before the valve seals replacement please help , dave