I agree with Cletis. A GMC was meant to have .020 lash on the exhaust. Remember you are dealing with a GMC, not a Chev. What kind of plugs are you using? I always had my best luck with an AC45 in a 270. The book may call for a colder plug, but they assume that the engine will constantly be at full throttle under full load the way we drove them 60 years ago. I doubt that you drive your like that. I would also check the points for dwell. Are you running the 270 distributor or the 228 distributor? If its a 270 larger truck engine, the distributor was not designed for a vacuum control. Using a vacuum control advances them too much.