That one BG400 should easily feed the motor and the nitrous. It should have a return line right off of it to go back to the tank. I'd use a -10 feed and connect your two regulators w/ a -10 also if possible, or whatever size ORB fitting it has on the regulator. Keep em close together for a cleaner look. Run -6 lines to the bowls and to the solenoids.

It sounds like you neutered the motor that was in the car when you bought it. It was a proven fast ride...why change it so drastically when it seems like what you're wanting to do w/ the car, was exactly what it was set up to do before?