Without external cooling you are limiting the possibilities greatly. I had some radical thoughts but those require some nice coolers to keep things in check.... That's the subject of another conversation, perhaps for that 71 E PCA racer
In your shoes, I'd opt to get some Nickies due to the improved heat management (material, extra fins, ...) and perhaps consider overdriving the fan (knowing that there is a possibility of cavitation).
I'd also consider some of Sherwood's heat barriers (anything to keep heat away from the engine) and perhaps some creative use of underbody airflow. In this app. I think squirters would not be a must but would be nice and can't hurt).