I would be cautious about running a high PSI cap. Especially with the 30 year old heater core and valve that weren't designed for those kinds of pressures. I would also be worried that the Mitsu radiator won't like being under that kind of pressure either. The overflow bottle is doing it's thing but if it's not returning into the system I'd check the overflow seal in the cap. I would run a real temp gauge instead of the guessomometer in the instrument panel. A few extra gauges are a good insurance policy. Eediot lights and iffy instruments will get you so far...