Frank, the beauty of many traditional hymns is that they are often structurally rather simple - most often the I-IV-V and relative minor are used. So learn two or three positions / inversions of each chord in your chosen slack key tuning, and move between these positions looking for the right melody notes. This is a great way to get more comfortable with a given tuning. An Em in standard tuning is also an Em in slack tunings - you just grab a different shape.
Open tunings also have one great advantage. If you happen to hit a wrong string, odds are it is still a note in the chord, so it is harder to go wrong.
If you are looking for accurate melody transcriptions and full arrangements, I don't have a good source for you. Best of luck. Make a joyful noise!