You may have to baste additional fabric to your backing so you have enough to attach to the leaders on the rack and attach the side clamps. LA requires more than DSM. I like an absolute minimum of 5" extra on all sides so I require the backing be a full 10" larger than the top in both length and width.
Also you don't say how you basted the quilt sandwich. If you spray basted with glue that may present problems but I can't say for sure. If you basted with pins, you will need to remove all of them before attaching to the rack. If you basted with thread, you shouldn't have any issues with it but it would also be advisable to remove those basting stitches.