I would not deviate from Hobie's recommended spacing of the factory cradles......having said that, what if you used a pair of 2" x 4" x 7 feet long, turned on edge and were "u-bolted" to the outside frame.....remove the round cross bars first.
This would give you a longer base so the cradles are mounted with the recommended 67" spacing.....you can easily add cross wise 2 x 4's to suit your needs.
I own a Trailex trailer that I modified with parallel 2 7/8" OD, heavy wall (~ 0.22" thick) x 7' long PVC pipes to carry my PA-14......the cross support spacing is 67" and the PVC "sags" to conform to the hull shape when loaded.......easy on/off and material cost is significantly less than the Hobie cradles......just another trailering/storage method.....