Like many 3A owners I had overheating problems which I eventually resolved as follows:
- Installed the cardboard duct
- Restricted the bypass
- Installed the high flow thermostat that Peter referenced
- Crimped the bars on the repro grille to increase the airflow to the radiator.
Each of these made an incremental improvement and collectively solved my over heating.
A PO had removed the mechanical fan and installed an electric fan. I also played around with different thermo switches for activating the fan and eventually used a non-standard temperature range switch