If you're going to transfer the first switch to the Z1 assembly I see no reason for it to not work. However it may be set for a different bypass pressure. I think the Z1 switch is around 2 psig, but don't have a book in front of me to confirm that.
I ride Z1's. I have a couple of '75's, plus an '80 KZ1000LTD. My lovely Sweetums rids a 1981 KZ550.
I have no clue about the thread size but I do know that switches like those are more of a flow indicator than a pressure switch. The oil pushes the spring disc out of the way so to speak and in doing so the ground path is interrupted.