Okay, had to go back and read the entire thread. You've replaced the fan clutch, thermostat, rad was checked out, base timing is at 16*, hoses are good. No vacuum leaks, it was mentioned, hope you checked that out. Not a lot left.
Only other thing I can think of is the vacuum advance. Is it working? Timing and engine RPM should jump up when it's connected, compared to disconnected with hose to carb plugged. Also where is it connected to the carb, near the base or up above the throttle plates? If it's up top I would move it down to a fitting near the base of the carb.
If you use a infrared thermometer check a few different places, front of head, lower on the block, center of head on the side. Then temps will vary some but should give you a good idea of what's going on.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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