You don't have to do anything, BUT disc brakes are a better brake, especially on the street.
I wouldn't drive a /6 car without discs and some handling upgrades. The only reason my GTS still has drums is because it is 1 of 271 340/AT 68 convertibles. But, I do have a disc setup if I decide to go that route (which I probably will).
Sure...you can run drums. People can also run a worm gear rear, cork clutch, solid rubber tires, galvanized pipe roll cage, convert to 6 volt electrics, remove seatbelts, put on a mini chain steering wheel with a speed knob, disconnect all gauges, use water instead of brake fluid, drill lighten all body panels so the car can whistle as it goes down the road.
These are all mods that will work....for a while.... **(*)
I wouldn' recomend any of them. If you never plan on putting your foot into it, run drums. If you never put your foot into it, why even go to a BB (mileage maybe?) Seriously...upgrade your brakes....definitely. Then upgrade the mill.
Have fun, good luck....all the info you need is on this board. Just ask for help anytime. Read the tech pages first for some good thoughts...
I'm finishing up the disc swap on my 73 Dart, and it is a slant six car (gonna stay that way too). I drove it and my Duster home on drums, and they won't see the road again until they have discs. It's really up to you, but I wouldn't trust my life or any stranger's life to near 40 year old parts.