I am sorry to say I don't have any personal advice so take this comment with a grain of salt.
I'm doing my rebuild and also concerned about drive train and modern speeds. The truck had '56 235, 4 speed w/ granny, 4:56 rear end. My dad last drove this truck regularly. He said he has achieved 80 mph multiple times and believes the truck will perform fine at 65mph
Either he is wrong or the truck could achieve that max speed.