Unbolt the fruit jar & bolt on a dual one from something newer with drum/drum would be the obvious way.
Why,it's a legit question. You will need to do a little brake line work and take the rear brake line loose from the T connection and run it to the master cylinder,,where you removed the rear line from the T add a new line and run it to the master cylinder,,I have probably oversimplified the process but it's not rocket science. HRP
If you walk into a parts wholesaler and order a Master cylinder for a 1971 Ford Maverick with non-power drum/drums, you'll be right about where you need to be for cylinder size. It also will still have the residual checks built in, so disregard anybody telling you you will need them. You will end up messing with the actuation rod a bit to make it all work, but the last of these cylinders that I have bought came with two different rods packed inside that I could mess with and make work. As HRP said above, you'll have to do some new lines for it, but that's not a really big deal either. If you ever upgrade to front disc brakes in the future, replace it with a '74 Maverick non power Disc/drum MC and life will again be easy...
as hrp said, just rip off the single and bolt up a double. then run your lines, easy deal be done in a couple hours with beer breaks
Cheap insurance. Safety with additional circuit and piece of mind. You may think about discs on front and a booster. I did a similar upgrade with my 390 FE 66 Fairlane.
I think the popcorn was intended for the "who needs a dual circuit M/C? I've run a single fruitjar for a hundred years...." crowd. personally; this is one of the FIRST mods I do. make 'em stop, THEN make 'em GO.