We can not express enough how much we recommend you purchase ARP products when working on your 2JZ head, regardless of power level goals. This ARP head stud kit is exactly what you need.
First, your stock bolts that come with your Supra are made to stretch. They are technically called TTY (Torque To Yield) bolts. Toyota (and all manufacturers) put TTY bolts in a head when manufacturing the car because they are the cheapest bolts available, and when you are using millions of bolts in a year even 10 cents a bolt adds up. Once these bolts are installed the first time, they stretch because they are made to. Once they are in your car for years they become rather worn out. When you take them out of your head they are stretched and should be thrown away (same with flywheel bolts). Never re-use the original bolts in your head. If you do you'll be buying another head gasket in a matter of weeks.
Second, ARP bolts and studs are rated by their tensile strength in PSI. Simply put, this is the amount of pressure that would be required in order for ARP hardware to sheer or snap. Although we can not rate a TTY bolt, we can estimate that their tensile strength is around 60,000 to 80,000 psi. All ARP hardware for the Supra is rated at an incredible 190,000 psi, about 3 times the strength of Toyota TTY bolts. All ARP hardware is made from a heat treated 8740 steel alloy.