Suburban/Blazer tank conversion / return line port

[D-n-Tyke]

Does anyone know how far over the fill tube flange goes inside the tank. I need to add a fuel injection port to this tank but am worried I am going to hit the edge of fhe flange inside the tank. I want to put one of those swag in port fittings but if it lands on half the flange I don't think it will seal right.

My plan is to put it right u der the vent pipe. Approximately where the red dot is.

Anyone have experience with these tanks and no where the flange ends.