Max distance between bars would be around 63 inches, if you put them on as far forward and back as possible (in those tie-down point sections). I've put one of Thule/Subaru extended bars in the max forward position before, so I assume it would be doable in both.
FWIW, my canoe carrying setup has the bars near the edges of the main section at around 48in apart. Feels pretty stable. Your idea might be better though