Your later backing plates are more desirable, the info you received on the wheel cylinders is spot on. The info about the location of the wheel cylinders at the bottom has a great deal of merit, next to impossible to get all of the air out of the system, unless you have access to a pressure bleeder. However moving them to the top opens a can of worms, the brake lines and maybe the bleeder screw will not like the location of the stock shock linkage. I wont go into the fix for this but will encourage you to do a search on this site with regards to Hyd Brakes, there is a lot to be learned. However I strongly recommend that you do not replace the the master cylinder with that single type. You will need a dual/split master. This improvement will give you a backup system, using the old single master you chance losing all of your brakes if a wheel cyl blows, with a dual master cyl if you blow one cyl you only lose half of your brakes