I like the GDAD tuning for a long neck mandolin. I find it works well with a guitar tuned in DADGAD. Many of the chord shapes used on a GDAE tuned mandolin are not possible to play on a long necked mandolin tuned as an octave mandolin, but dropping that first string a whole tone just seems to make everything easier.
I'd think using octave mandolin strings for the first four strings and guitar strings for the 5th and 6th strings would be a good solution to arriving at DAGDAD.