PSI isn't a good way to measure how much power is safe. 18 PSI is much more dangerous on the tock CT26 than on say a SP61.
HP is a much better measure. Some people have touched 500hp safely on fresh stock internals. That is some thing you will not be able to do safely on a 200,000+ mile motor.
A little more info on your car and what you’re trying to accomplish would help in determining what would be safe for you.