There's an article on this on how to do this under "SV2x Rear drum to Disc conversion" under the Articles section.
Did it to my Caldina (same rear hubs altough with ABS)
Parts from wreckers cost about $200 + $65 for new disc brake pads.
Note: on my conversion needed to use the stub axles from the disc (the drum part sat differently) & make sure you get the complete handbrake cable (at least to where it joins from 2 into 1) the disc brake ones are longer with different ends.
Made a hell of a difference to the braking, although I had already fitted GT4 calipers to the front so the brake balance was out.