The machine will not know the difference between the board and the sled. The machine will only measure the length of the sled. It looks like you have the extra 3 1/2" on the end of the sled and so do not need extra blocks.

There are many choices for how you tell the machine where to carve. One way would be to make the board in Designer the same size as your cedar plank and use jog to corner at the machine.