From my understanding, doing the same thing myself. That you will need a set of 400 motor brakes from a 400 out of a Fire Bird set up. The 403 and 400 share the same motor mount (on cross member). From what I found the motor mounts (clam shells/on cross member) are bolted in the same spot. If you bolt your brackets to the engine and your motor mounts also, with the engine and transmission in the car you use the transmission cross member to help find the right placement of the motor mounts on the cross member.
If anyone has different info or if this sounds correct please feel free to help out.