Help: misfire issue

[TRex]

Hey guys so i got a question. I have a 1978 GMC c10 short wheel base and i came across a problem and wondered what yall thought...keep in mind that everything on it is new (GM 350 as well), well all of a sudden it started vibrating worse than normal when i shift into reverse or drive but not as bad when its in park. So i changed out the spark plugs and put new wires on and cylinder 8 the plug was black but the rest was fine...so anyway i replaced them all and fired it up and it pops every once in a while in 3rd gear (3 speed) and its still rough nothing has changed...i gapped it at .045 thats what it says on my truck to do but is there any other reason why it all of a sudden started up? Ive been told maybe its bad gas

 

[Ricko1966]

Pull the number 8 plug wire at the cap. With the truck running set the wire back on and pull it back off put it on pull it off. Pay attention to what the engine does no change on or off , then you defiantly have a dead number 8. Next step would be check compression on number 8 , you can check the other 7 if you like but no point if number 8 is extremely low.