For things like sending units, I'd just go your local auto parts house. I use NAPA a lot...they handle several universal sending units that you can get in different models depending on the depth of your tank. Many times, the easiest thing on the filler neck is to cut one out of an old tank, and braze, weld, or solder it into your new tank. That gets you the proper end for vented caps etc. Do note that if you do cut one from an existing tank, do it very carefully. No flame cutting or abrasive grinding, and even sawing with a jigsaw can create a spark. Best precaution is to purge all of the air (oxygen) from the tank and replace it with an inert gas such as argon or CO2 from your mig welder. Once you're sure that no oxygen remains, you can cut it out with a saw.
Good luck-
tjs