Boost is controlled by the waste gate, the looser the spring the less boost/force it takes to open and stop boost from building. Most usually have a 4-8psi spring in them. HP per PSI VARIES a TON, it's completely dependent on the motor efficiency and turbo setup efficiency. Some make 12hp per PSI, some make 25hp per PSI. My setup makes 27whp per PSI.
I'd hold off on the turbo till you can afford a better block. That motor is gonna make a ton of power with a 70mm hanging off it.
fmyth, someone couldn't tune your car or you spun it past 6000rpm too much. My car made 488whp/504wtq on 13psi. I have put over 10,000 miles on my stock '91 bottom end and it's held up fine.
'88 LX -New mill coming soon.