Taller impeller gives approx 15% greater Vol, this is sold as a "280ZX-T" pump.
The springs are all standard.
Nissan Comp did an Option spring, that fits inside the standard spring, to cause pressuer relief valve to stay closed until a higher pressure was obtained, this will put extra wear on drivespindle gears.
An old simple trick, pack approx 4-5mm of washers into the large hex nutted screw, cap, this puts a bit more force onto the spring, increses press relief opening by around 6-8 PSI
Use feeler guages to check if impellor is within tolerances, and for end float.
You can use a sheet of 1200 grit wet/dry, to polish out any scoring on ends of housing, this can reduce the end float a bit, which increases vol/pressures. Place 1200 grit onto a sheet of glass, use a figure 8 motion and a little 10 weight sewing machine oil.