Hi Comtechy

Not sure which two planks you mean but:

Take your time with each plank and treat each plank as an individual job.

Lay the plank from the rear of the launch to the front and mark the overlap as best you can.

Remove only a little at a time and test fit. Do this until the plank lays in the correct position.

Do this for each remaining plank.

If you can post images with your problems then please do so, it makes it easier to help.

Have a look at others build dairies as well as others have similar problems and a solution can often be found by reading others experiences.

Hope this helps