Do you have the small rectangular resoviour master cylinder?

Converting to ABS will be very hard, I am sure most if not all models with discs all round were ABS equipped?

Fitting the ABS type master cylinder with the big resoviour will be very difficult as there are alot of gubbins, you have the valve block, loom, ECU etc just to name a few things.

I don't think you will have excessive travel, but I would imagine your current M/C may not be man enough to operate the rear discs, the ABS system works under high pressure so can do so.

You also will need a sensor on each wheel (hub) I am not sure if models without ABS have this provision on the hubs.

You will also need a different handbrake cable.

Martin