Loosened exhaust pipe at manifold. No apparent change. Actually cut off muffler to eliminate that as a possibility. Turns out it was a straight through muffler with baffles/louvers lining the tube. It was clear. Rechecked spark plug gaps as my son-in-law did those. They were set between .04 and .045. Set to .035. Adjusted the jet on the carb 1/2 turn out. Took it for a drive. Still starts bogging down after a couple miles at 45 mph. Sounds like some of the cylinders aren't firing. It seems to run fine just driving around the neighborhood.
The butterfly is stuck in the closed position to the intake.
Any ideas appreciated.

Thank you

"Do not walk behind me, for I may not lead. Do not walk ahead of me, for I may not follow. Do not walk beside me either. Just leave me alone, you're starting to freak me out."

1957 GMC 150