Before anything, if you want to run 15psi you should get rid of the check valves and buy a GM 3 bar map sensor. The stock MAP will code after it sees 10psi. Just like silver said, get an EMS. If you can't afford one you can use a free EMS like turboedit or uberdata. Only thing you have to do is chip your ECU. The reason why you need a 3 bar map and not check valves is because the EMS is going to read MAP sensor voltage, and the check valves will screw up the reading.
Do they make Uberdata for OBD 0?