I wouldn't worry about the standing water in the circular depression over the sending unit location. I put mine on about 12 years ago, and I've never had a problem. I think that anything you do to try to totally keep water out of there is more likely to trap water in there.

If you're concerned about the metal ring and other parts rusting, I'd just clean them up good and give them several coats of Rustoleum.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.