I saw this old post and thought of a couple of improvements that could be added.Jegs sells a 5psi pressure activated switch you can use to either open or close a circuit' . it could be plumbed into the fuel line by the carb and used to run a relay that shuts the electric pump down at 5 psi fuel pressure and above. Also to make the line from tank to mechanical pump less restrictive, I would try putting a T fitting before the electric and have the electric pump on another line with a one way check valve in front of it, then join the two going to the carb. A bit of extra complication, but if results are worth it to you,,,,,,,,,