To be honest the ABS master cylinder would have been fine for the job, theres not really any difference between the two in terms of operation. And if you have an RST the non ABS item you have bought will be smaller than the original.

Being realistic with you i would say don't fit the rear discs they arent really worth the work involved.

If you want to go ahead with it though you are best to keep the ABS master cylinder, and you need to replumb the system. Currently you will have a diagonal set up meaning one front and one rear caliper are operated from each piston within the master cylinder.

You need to run both front calipers off one outlet from the cylinder, and both the rears off the other. The bias valve simply needs to go in the pipe from the cylinder to the rear of the car. The piping alterations should be fairly easy if you keep the ABS cylinder.