I just completed a hydraulic clutch conversion on my 82 GPZ750. It works like a champ. I bought the slave cylinder from american classics. It is supposed to be a drop in for KZ750, and probably is, but not for a GPZ. (found that out the hard way!) The spacing on the bolt holes do not line up, but a retainer strap can be made by tapping and utilizing a couple of the protrusion on the inside of the cover. You also need to grind down the cover so that the socket is flush with the three raised tabs around the socket. Once this is done, you will have ample clearance. I trimmed my push rod 5/8th's and it was right on. I used a 85 Honda CB650 clutch master cylinder (yah, I know) bought for $30 bucks off ebay, as it had square reservoir not angled and hose which was maybe 2" long, but that extra slack was easy to take up in the routing, and the hard line section gets you a lot of clearance down the frame. Gave the bike a good shake-down around town with no problems at all. The only disadvantage to using the honda master cyliner is if you want to retain the start safety switch, you will need to cut off the stock switch, and crimp terminal ends on to use the switch on the honda set-up. I just tied mine off out of the way. The lever is a little different also, but most people don't even notice. So all told, it cost me $200.00 to do the conversion. Maybe not the cheapest way, but it worked for me.
current bike:
1982 GPZ-750
past bikes:
1978 Z1-R
1982 KZ-550, 615 kit, cams, head-work
1978 GS-1000 drag bike (younger days fun)