not 100% sure and i dont want to waste your time either, but when you say not starting do you mean, no cranking, no compression, is the engine turning over?
you say you are using rb25 engine and loom, what about the computer?
double checked all electrical connections? and not by looking but by feeling them?
is the engine used in the conversion new/secondhand/rebuilt? condition of engine, block and head?
hope this gives you some kind of idea, or even helps you.
cheers.