I would obtain a steel nipple of the same size and thread as your brass nipple. You can cut longitudinal grooves in the steel nipple to help the rust find a way out as you use this nipple as a thread cleaner.
The brass nipple will then go in much easier and you will have a better feel for how tight is tight enough. I would not use any sealer if you can clean up the threads.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission