So, here's a little educational stuff about harp guitars. Most harp guitars have either 5 or 6 bass strings. A common tuning for 5 strings are G, A, B, C, D, low to high. The D is the D below the 6th string. Now you've got nice low resonant strings to really fill out your chords. Imaging an A chord with an A an octave below the 5th string, or the rumbly G chord you can get. Michael Hedges used G, Bb, C, A, D for the song "
Because It's There."
For 6 strings, F, G, A, B, C, D is a pretty common tuning. Stephen Bennett came up with a nice combination: G, A, B, C, D, G (equivalent to the 3rd fret 6th string). 7 strings are also popular, with the strings E-D.
The Gibson with 12 bass strings are usually tuned chromatically: E, F, F#, G, G#, A, A#, B, C, C#, D, D#. Good lord, I could never keep track of those strings. Gibson 10 Strings were tuned A#, B, C, C#, D, D#, F, F#, G, G#.
I think 5 or 6 strings would be the most useful.