The receiving unit of the launch arm is threaded, so mounting the valk is just a matter of "tightening" the bolt into the threaded receiver.
You will be able to incrementally check the height/position of the valk by temporary mounting the boosters. They don't need to be permanently attached at this point.
In fact it is very important NOT to mount the boosters permanently until the valk is in the correct position. You will not be able to get the boosters under the launch arm correctly otherwise.