Bigger sway bars.. is it worth it?

The front bar keeps the car from "rolling over" in turns. The bar you have may look sissy and have a smaller diameter than some earlier bars, but don't be fooled. The K-frame and disk brakes changed with the '73 models, so the bars do not interchnage. The shorter length of the later bar makes it stiffer overall. The drawbacks to a front sway bar are a slight increase in unsprung weight (rougher ride) and understeer. I agree with the recommendation to keep the stock bar and add a rear bar. If you can find one that mounts on the body (a'la A39 Dart), you'll be better off than one that mounts the bar on the axle.
I installed a K-frame & bar combination like you have in the Demon. I use .830 diameter torsion bars and the stiffest affordable gas shocks I could find. All the rubber in the suspension has been replaced by polygraphite. Combined with the cop car steering box, the car drove nice.
When I added the rear bar, it balanced the car better and made it corner flatter, too. The rear bar is mounted on the body. I had to fabricate mounts to connect the ends of the bar to the axle. The rear is soft sprung/stiff damped like the front end. It's no Porsche, but it is fun in the twisties.