What's a safe psi on a stock engine?
It depends. Years ago when I bought my supercharger tuners were saying 8 psi, or 380 rwhp. Top tuners were saying that in all the car magazines. ProCharger published a chart that said 8 psi on Romeo motors and 10 psi on Windsors. Why more on a Windsor? They have a lower compression ratio.
Back then a New Edge was still fairly new. They were just out of warranty and a lot of people were modifying them, installing superchargers, and pushing the envelope. The general rule of thumb became 10 psi or 400 rwhp on pump gas. But risk takers disagreed. They said they heard about 13 or 14 psi and some posted outrageous claims of 500+ rwhp. They did this without telling the entire story.
People were also blowing up their motors. The more boost you add the less margin of safety you have left. One guy bragged about 13 psi and 415-420 rwhp. His motor lasted 9 months. Another bragged about 14 psi and 430 rwhp. His motor lasted 6 months. Some guys built dyno queens. I remember one posting that he was running 13 psi when the car was tuned but he refused to run it past 5500 rpm when actually driving it for fear of blowing it up. Some built garage queens driving them only 1,000-2,500 miles per year.
The general consensus is that 10 psi or 400 rwhp is reasonably safe. If you are running 10 psi on a completely stock motor you should be near 400 rwhp. That is SAE corrected which is a more common standard than STD. 9 psi will make 400 rwhp using STD correction. (reference my signature).
Do some get away with more boost? Yes. Does everyone get away with more boost? No.
If you want more than that then you need to make the motor more efficient by installing supercharger cams. They typically add 30-40 rwhp without running higher boost. Want that near 500 number? You need to put in big injectors and run E85.
You need to decide how much risk you want to take. I never recommend something that I wouldn't run myself that is why I say 10 psi and 400 rwhp is reasonably safe on a stock 4.6 2V.
What pulley should I go with? Maybe 3.85?
You need to get your problems fixed and your tune right before you start adding smaller pulleys. What is your goal for your car? The old saying is "Plan the build, build to the plan." If you don't know your goal you are just throwing money at it.
There is another old saying, "fast, reliable, cheap." Pick two. Fast and reliable isn't cheap. Cheap and reliable isn't fast. Fast and cheap isn't reliable.
My goal for my car was that of a reasonably fast, fun daily driver. I consider 12.7/111 mph to be reasonably fast. My car was hardly a garage queen. Before I retired I drove 20,000 miles per year and I hit 6,000 rpm almost every time I drove it. It is fun to drive.
I still drive my car 7,000-8,000 miles per year. It has 210,000 miles and the last 130,000 were supercharged. I change oil twice per year. It using around 3/4 quart of oil between oil changes. I would say it has been reliable.