Re: 1966 c20
Not trying to talk you out of the 350 at all but the 250 should easily push that truck highway speed. Something else is up causing that 45 mph top end.
The 230 in my '55 GMC will run down the highway 75 mph no problem but I keep it 60-65. I need to get a tach someday. The slapping sounds like something in the drive train.
Just guessing that the speedometer cable probably needs cleaned and lubricated. When they get sticky they kinda wind up and release causing a very erratic reading. Truck looks great, thanks for the updates.
DAC
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Doug Crawford
Rapid City, SD
1970 GMC K5 Jimmy Mom drove 30 years
1972 GMC C2500 owned since 1979
1955 GMC 100 driver-project
2006 GMC Yukon Denali---wife's truck
Hope to have a '60 GMC Suburban again someday
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