Well, that's ertainly one solution -- in fact, that's what I did when I installed the polyethylene tank in my CJ-7 about 12 or 13 years ago. The previous owner had totally removed the gas tank, and used a pony beer keg in the bed for a gas tank for sand drags. When I installed the plastic tank, I had to get a new sending unit, etc. I just used a "U" shaped piece of small hose and connected the two "valve" outlets to each other, and used a vent-style gas cap. Seems to have worked well for over a decade.

Regarding the non-tight fit of the sending unit. I don't know what to say there -- the new parts fit perfectly on my tank, and the sending unit works good. Indicated low for a while, until it got fully saturated with fuel. But no leaks of gasoline, and no entrance of water into the tank, despite some deep water crossings. If you don't trust yours not to leak, I would think that RTV would probably seal it good enough.