what do you think or does your engine builder (or you in this case) think the power will be, HP? I'm also going to guess that you are staying naturally aspirated?
I'm going to shoot in the dark and for a 403 naturally aspirated say 50# to 60# and you will have some breathing room.
for a 403 boosted, I would grab 850cc or 1000cc injectors (80# and 95# respectively)
contact FIC Injectors