The sand storms will continue until the sand is heavy enough to settle quickly. That doesn't happen until the sand grains are coated with bacteria. That doesn't happen until after the cycle has been completed. You are going to need more rock in the new tank anyway so add some base rock and then add some of your LR
from your 40.
I never suctioned my sand bed. Today I can have a power head fall and create a major sandstorm that will settle in about an hour. That can only happen AFTER the sand is cycled. You can suction the top layer over and over again. In about week of suctioning you may be able to stop if the that layer of sand has had time to get coated with bacteria. Otherwise you will end suctioning all of the sand out of your tank.
Using the water from your 40 is fine, BUT a) it won't fill your 75; b) it will not cycle your 75; c) adding it will cause another huge sand storm until the new sand is cycled.
Bottom line is that you need to cycle the 75 before you transfer the majority of your 40 over unless you don't mind living with a sandstorm for a week and hope yourfish can find food and survive it.
Take the time to do it right. You will be happy you did.