I have owned 3 1966 Chevrolet pick-ups, and each one felt somewhat different on the road.
My C-30 with a 250 and a 4 speed was OK up to around 60mph. Beyond that speed, it did not feel right. I took it up to about 70 once, and it was really scary.
My first C-10 also had a 250, and a 3 on the column. It would cruise safely on the interstate at 65-70mph. Going any faster did not feel right.
My current pick-up is another C-10, and has a 283 and a Powerglide. I have not owned it too long, but it is comfortable going 65-70mph. Even though it is capable of going a bit faster than the sixes, it probably will handle like the others at higher speeds. Therefore, I will not go any faster than 70mph. It is just too dangerous.