For my whitewater kayaks i have the back mount about 6 inches in
front of the most forward bolt heads for the tops hinge. The front ones
are just behind the skylight. When i open the skylight it misses the
crossbar by about 1/2 inch. This gives the crossbars a spread of 4 ft. 3
in. If your talking recreational kayaks, Most of them will fit in this
position. If your talking sea kayaks  then you'll have to move
everything forward because of the length of the boats. One word of
caution, carry your paddle INSIDE the vehicle.