318 - High Oil Pressure

Oil pressure is a function of the relief valve and relief spring. Standard spring is 45 psi and HP is 65 psi, using 20w50 or 10w30, your oil is not the issue. The rule of thumb is 1 psi per 100 rpm, and relates to keeping a wedge of oil on the rotating journals. I doubt you will make power to 8,000 rpm, so it is a waste. It can also balloon oil filters and crack blocks if it gets too high. When I had to "touch" everything, I broke the edges on the relief valve in the oil pump and it acted as you describe. I'd replace the oil pump or relief valve and spring at least. Pressure from the breather would be lack of ring seal.