Wish I’d dialed into this thread sooner. I had a ‘57 with the 283. All the good stuff has been well covered here.
BUT I didn’t see this point mentioned. Do you know the compression ratio for this engine? I’m not sure it’s original based on modification to the radiator support where they’ve pushed it 3” forward. If it’s an original compression ratio, it will likely need premium fuel, especially with proper timing. In my experience, the 283, which was originally an engine calling for leaded fuel, it would easily experience preignition at temps above 140F. Advancing the timing made it worse, and until I figured it out had to run it a bit delayed. If you’re getting some popping, increase the octane while you get the settings dialed in.
these vehicles had a solid metal 4-blade fan with no clutch and no shroud. The fan-radiator gap was crude, about 1-1/2 inches if I remember. The idle rpm spec for the automatic was 450 rpm in gear (no kidding, I had the FSM). Iirc, the manual was a higher rpm, I think 550 if memory serves, at least with the 283.
and oh! These were not like the SBC’s of the 80s and beyond…. it was normal to heat up at idle. And the temp gauges were not buffered, so you’d see the real temperature swings that are hidden electronically in all modern vehicles. especially if the engine has finished a hill climb with heat built up, and then stopping, it’s harder to get it out and the needle would move from 40% to 65% on the gauge. So…. Some of what you’re seeing, to an extent, was period correct.