vacuum for the wastegate actuator control must be taken from before the throttle body. doesnt matter where, you can just drill a hole in a convenient place in the intercooler piping and thread a fitting into it for get one welded on. closer to the turbo compressor the better though.
running off the inlet manifold will cause all sorts of spiking issues
EH@B, I am not upto date with your problem but are you saying boost is too high?
You can not lower the boost less then what the internal wastegate is set to.
If it is adjustable then extend it as much as you can (you must leave at least 2 threads on it).
If its a fixed arm then you won't be able to lower less than 12psi i think it is on 200sx turbos.
stock for the 300zx is 9.5psi.
Have you confirmed that the actuator is good? It may be that it is not opening?
if you suck on the vac hose intake on the actuator and then put your tongue over the end without letting air in, it should create a vacuum suction and stick to your tongue. if not then its toast (dead).