I have a 454 BBC. At WOT the fuel pressure gauge reads about 5.75psi. I have a holley blue pump. If I introduce a 125hp nitrous kit (NOS powershot), can the same fuel pump carry both the motor and the NOS?

I sure wouldnt take a chance. Lacking fuel with nitrous kill motors fast!!! If you can do it, buy another Holly fuel pump and have a pump dedicated just for the nitrous. It is cheap insurance! That way, when you want to spray even more H.P. :D , You'll be safer. I had a 454H.O. in a 70 Chevelle with a dedicated system with some safety switches, and sprayed it with 275 H.P. many times and no problems.
good luck!

just wondering...
where do you T off for the electric pump?