The RN 41 was available with discs (I had one), get the disc set up and bolt it all on there, they work well. edit/ you may need to swap over the whole stub axle assy.
The brake set up of the 4WD will not fit AFAIK(you would need to measure up and check it out).
You may be able to use the discs off the BA Falcon (same bolt pattern) and PBR calipers, but again you will need to check it out as we are not all of the same ability.