Clutch City

Hello all, I recently connected the code reader to my 1999 4.3L Chevy Silverado 1500, 2WD. I would get a code P0300, Multiple Misfire. So I did all the norms check the wires and spark plugs, distributer, injectors, etc. Found several injector hoses were cracked in the throttle body housing. So I changed the spider assembly out with a refurbished one. I used the Seafomed with good results. The truck ran great for one day, but started missing the next. So I hooked up the code reader again and got a P0306 code and P0440 emmissions. I pulled the number 6 plug and low and behold anti-freeze in spark plug well. What would this be an indication of? There is no water or antifreeze in oil. I have good vaccum, decent compression.
Could this be a bad head gasket, valve spring or what? Help!