Well it’s likely the Venturi nozzle is in line with the the outlet so maybe able to be drilled bigger.
Does radium have the sizes written down for the pumps used?
Someone with way to much time and a spare fuel regulator could fit a factory pump to the cradle out of the car and see how long it took to empty a bucket from the Venturi second tank side inlet. Then repeat with a bigger pump and it emptied the bucket faster, guess very likely. Then Drill the nozzle size bigger a little bit at a time until it was close to the original time.
Ball valve idea shouldn’t be to hard with plumbing fittings. Just the space to fit it could be an issue.