Your current master cylinder is set up to send pressure to rear drums.
It will work with rear discs but not properly (I still have my drum brake master, I have had rear discs for 2 years) I plan to swap them out very soon.
Disc brakes require a different type and amount of hydraulic pressure then drums, so if you want it to be right you will need the rear disc master.......
I have messed with the front/rear biast on my master cylinder tons of times (turning the plunger in and out) trying to get it just right (75% front, 25% rear) and I have been unable to get it perfect (although now it is close).
Also we have noticed the rear discs drag slightly when using the drum master, causing premature wear on the rear pads, and also effects gas mileage and rotor wear.....
details, details....
yes you can get away without it, Rick has been running his discs forever with the stock master, no problems.
Brett (who wrote the conversion article) has the same issues I do, I learned this stuff from him, he helped me put the disc brakes on my truck years ago......he also has rear discs with the drum master on two trucks, but agrees to get it right the master must be swapped out........