I think trying to use an off the shelf in tank fuel pump should be nice. The fittings for the feed and return are going to have to be under the seat which will cost you a little room. You will also need to route those lines through the floor at some point.I am working on my fuel tank design. It will straddle the tunnel and occupy the space beneath both seats. I plan to have a sump cut through the floor on the passenger side because exhaust is on the drivers side. I can drop it 6" below floor because I used 2x6 frame rails on the belly of the frame and I will protect it with some kind of gusseted skid plate. I will run a hard line from drivers side to the sump for complete fuel transfer past the bump for the tunnel. I need a complete drop in tank fuel pump setup. I would like it to include sender unit for gauge and fuel return line hookup. I need minimum 60 psi.
I have used Walbro in the past and have found them to be quiet and reliable but I have not found a complete drop in universal kit. HAS anyone used or seen one, or maybe suggest a good alternate? Quiet is important as it is essentially in the cab.
The fuel level sender, if it is a lever arm float style, will likely need some modification since the tank shape vs capacity is going to be a little odd. I would imagine that you would need to shorten the rod so that the float travels from the top to the bottom of the MAIN part of the tank to get a somewhat accurate reading. I would just ignore the fuel capacity of the sump since it will probably be less than a gallon.
I would see if you can find a good parts house to pull some pump assemblies off the shelf. Using a GM sending unit will most likely be best. I would start with all the factory applications for your engine and go from there. I would imagine that there should be a factory pump unit that is about 9" deep or so. Most of them also have a little adjustment built in.
Finding a way to attach the pump assembly to the tank could be interesting. Goatbuilt uses a factory GM pump assembly on his Ibex buggies I think. That would be a good place to start for the mounting ring stuff....