[T Herder]

You can use the 5/16 sending unit with no problems. My 305E is fuel injected with 5/16 supply and return lines to the stock tank. It won't starve. Here's the procedure I wrote for another forum to add a return line to the stock tank.

Sand an area of approximately one inch diameter to bare metal on both sides of the new units mounting plate. Drill a 3/16” hole in the mounting plate and remove any burrs. Support the mounting plate on the internal side of the plate with a 9/16” six sided socket, the six sides towards the mounting plate. Take a 5/16” diameter tapered drift punch and drive it through the hole to form a tapered flare recess. This recess will be on the outside of the plate. Using a tubing bender, bend a 5/16” steel tube to parallel the fuel pickup line ending at the same bottom level BUT returning to the OPPOSITE side of the tank. Bend the external side of the return tube down to parallel the feed line. Carefully work the new return line through the 5/16” hole to what you THINK might be its final position. Then try fit it into the tank to confirm it will install with no problems. Once this is accomplished, Silver solder the 5/16” tube into the mount plate using 45% Silver solder.
If you’re hesitant to try this, practice first with a piece of scrap sheet metal and scrap tubing of the same gauge as your new replacement mounting plate.
Best regards,
T Herder