If the racks are OK for river kayaks, then adding bow and stern lines will help in securing the longer sea kayaks. if you feel you need more support under the boat, put a 10 to 12 foot long 2x8 securely on your racks, and make cradles on that 2x8 to fit your kayak. It will be something like a strongback, but with cradle cut outs instead of building forms mounted on it.

Steel eye bolts in the 2x8 will hold the bow and stern lines, and you'll be able to use slightly lighter lines to hold the kayak to the cradles.

PGJ