Those I/O's will get 5 mpg, so they don't NEED as big a tank!
I'm curious, since your Seafari is an I/O, does it also have a galley seat?
The reason the vent is at front of tank, is because if you're refueling at the dock, you'll be standing at the BACK of the boat, so there will be an air bubble in front of tank. If fuel fill is on stbd side, the vent should be on port side of tank. If you move vent to the back of the tank, you won't be able to fill tank completely because of the air bubble, unless you only fill it on trailer with the tongue on the ground!