340 Overheating Help

For starters get a good gauge on there. Factory gauges can be hit and miss, especially 40 years later.
Keep in mind a fresh rebuild usually will run a little warm until broke in.
There is a long list of things that can make a car run hot.
Going to have to start checking off the list.
Examples
Running lean,to much timing,clogged radiator,radiator cap,to small radiator,dragging brakes
ect.