There is lots of discussion on this, including a similar thread in the last few weeks... maybe exactly the same... to be brief usually folks go wider than center to center how much is personal preference (search old threads).
66" would be what I'd put on there with those two choices...
Yes you will have more weight bearing on the cross bars than the side rails, but it will redistribute out quite a bit when you load the boat (depress the tubes and spread weight to the frame rails). It will not stress the frame to do this, the most commonly discussed drawback is wear from the frame on the boat but in my experience the opposite will happen. The frame rails will wear, leaving black marks in the form of oxidized aluminum powder. Soap, water and a Mr. Clean Magic Eraser and the rubber it will look new, while the ends of the cross bars and the low pro's will show obvious wear (loss of annodization at first, then the AL itself).
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae