The stock disk/drum set up on the e-bodies isn't really that bad, better than a lot of cars from the era, and even later. Sure, its not like having a new car with ABS, but if you feel like you're not stopping well, then something is out of whack. My Challenger with the stock disk/drum set up had no issues locking up all 4 wheels.
I'd pull the drums and take a good look at the shoes, check the hardware, grease anything that doesn't move freely (keep it off the shoes), completely purge all the old brake fluid out of the system and replace it with new fluid, and make sure everything is bled well. Brake fluid does not last forever, and it will absorb water over time. When I bled my brakes the first time, the brake fluid I got out first was a nice shade of dark brown.
Apparently maintenance wasn't big on the previous owners list. Old brake fluid will give you a lousy pedal, rust the lines, and cause brake fade at much lower temperatures than you'd expect.
Also, if there are any seals leaking in the back the shoes could be contaminated with oil or brake fluid, neither will help you stop very well. The disks up front should be pretty easy, but same deal, check the pads for contaminants and see how much life they have left on them. Also possible that an earlier owner went cheap on the pads/shoes, crappy pads and shoes will give you less than stellar braking. You don't necessarily need semi-metallics, but stay away from the econo pads and shoes.
Also keep in mind your tires are a big part of your braking performance. When I went from 225/60/15's and 245/60/15's with crappy tires to 275/40/17 modern performance tires my braking improved a TON. Those big tires really work the brakes now.
And if the stock stuff still doesn't cut it, keep in mind that Ma Mopar did equip the F/J/M body and police cars with 11.75" front disks and 11" drums. Not as much of an upgrade as some of the new kits out there, but a lot cheaper, and an easy upgrade. You may even get lucky and find all those parts in your local wrecking yard, they came on quite a few models in the late seventies and early eighties. That's the set up I currently run on my Challenger, and it stops pretty darn well.