I don't disagree with any of the above. Some aftermarket conversions (at least for 55 birds) include different rear cylinders, supposedly eliminating the need for a prop valve. Didn't work well in my case. You're going to want to start with a disc/drum master, then you have to consider residual pressure valves, and adjustable pressure valves. Some factory combination valves include some or all of the above.
If you're doing this on your own, go look at the aftermarket company kits,
http://ssbrakes.com and
http://www.abspowerbrake.com
Are two that I've used.
You don't need to buy a kit, but look at what's included. You'll need a good double flare tool, and maybe some other tools to make up the lines right. All doable, but brakes are not a place to make a mistake. If the old brake lines haven't been replaced, think about that. If you didn't do the hoses, they're next.
My bird went from drum/drum, to disc/drum, to disc/disc, over the years. When you make an upgrade, the weaker parts show up.
Well worth the effort to do it right.