'66 Valiant Running Hotter than Expected

Mattax said
"All in" really means little without the rpm info. Regardless I stand by my earlier posts here. Its a sidetrack.
Investigate the basics first.

Only other basic to add would be to test the cap and system with a pressure tester.

Done. Cap starts to leak off slightly at 13 lbs and is fully blown open at 16 lbs. Coolant level is where it was 6 months ago and has held the same pressure for the last 30 minutes.