In the past, I had a pinhole leak in my radiator (not leaking any viewable fluid) and had a similar experience. May need to get both your radiator and heater core pressure tested.
I used both cork and rubber gasket but still had small leak issues (possibly pan slightly bent). I friend told me to ditch the gasket and just use permatext without the gasket. Worked flawlessly! No leaks! And cheaper.
My experience with the HEI distributor (ACCEL 59130 with coil) is you must use the vacuum advance stop plate. Initially, I did not install the stop plate and the first time I hit the throttle, the results were not good. Vacuum advanced with no limit and did not come back down. I ended up...