Not really, unless you are racing with some class restrictions. If you are racing without rear brake rules, you might as well go for 4 wheel discs.
The front brakes are doing the major job in stopping the car. Just make sure you have good shoe material and drum id is within limits and see that wheel cylinders are in good condition (no pitting or worn seals), replace with good quality brake fluid yearly or so and bleed them regularly. Of course if you have a cheap source for backing plates and drums and stuff or you starting from scratch, why not.
Although, my rear lugs were rather hot even after 5 min on the paddock after 3 laps when I tested tires, the rear brakes do something, too.
Well, it just isn't a wider drum. You must also replace the brake backing late, which is offset toward the center of the car more. This is the same brakes as the 1st generation Ford Econoline van...the one with the engine between the front seats.
Because braking is such an issue with the early Mustangs, I put the factory front disc setup on my Hi-Po (an all drum car) during the restoration. After getting the car on the road I still felt it was sadly lacking in the stopping dept. So I did do the 2-1/2 drum conversion on the rear. Gotta tell ya, it was well worth it in my estimation. While bigger drums in the rear may come into their own in racing applications, it darn sure improved not only my cars stopping power but just as important my piece of mind. I'm less apt to bite a hole in the seat now when confronted with a sudden need for less speed. Probably an even better approach would be adding a power booster to your existing brake system.
I did this swap on my 66, I used parts from a 89 Crown Vic. I used the original wheel cylinders, purchased used backing plates from Junkyard Dog for $50, then bought the rest of the parts from a local store.
The only issue I had was the diameter of the hole in the center of the drum. I have an 8" inch rear, the hub center on the axle is smaller than the hole in the new drums. I went overkill and had two rings made to be sure everything fit perfect, but now that I have assembled everything I discover they really were not needed.