2/3 of governed rpm was considered the rpm for fuel economy. However if the engine is in good shape, running at the governor doesn't hurt them. We never had a Chev, but when we used GMC to actually haul loads of gas, we even bumped the governor up a little or removed it. As long as we had enough radiator to keep them cool, they were fine. We used to get radios out of Ford cars which were also 6v neg ground with heavy 6x9 speakers and that did wonders for drowning out the engine and wind noise.
