The shower tube is basically already off of the top of the dishwasher. See below:
Why do you suggest taking off the roof spray arm BEFORE replacing the tube?
The reason I'm asking is because the two screws holding the roof spray arm are severly corroded.
I can remove the screws using a grinder and if I do that then I suppose the roof spray arm will be able to be released from within the top of the dishwasher. I can then get two replacement screws to reset the roof spray arm.