You'll need:
1. A boost elbow - this restricts the compressor output pressure to the wastegate actuator. In other words, it allows the turbo to produce higher boost levels before the wastegate opens.
2. A boost fooler - this is an electronic device that goes in series between your manifold air pressure (MAP) sensor and the wiring harness plug. Without this, the ECM will see the higher boost pressure and (1. ) defuel to lower the boost pressure and (2. ) set an overboost code.
If you don't want to buy a "box", Practical Solutions used to offer a reasonably-priced boost fooler.
Rusty