I have bars from Firm Feel on my '68 convertible barracuda. I also have heavier rate front and rear springs in mine, plus the full chassis stiffening kit from US Cartool including weld in subframe connectors and Koni shocks on all 4 corners.
If I had to choose one system or the other, I would install the subframe connectors and chassis stiffening kit first, but its the package of all these components that transform these cars into near slot cars with a smooth compliant ride on the open road.
Some guys like to slam these cars down by lowering them, but I prefer mine at the factory ride height in the front with a slight rake that a slightly tail high ride height provides. It works very well out in the real world by keeping my headers, oil pan and transmission pan higher up and further out of harms way. My car will still out handle the majority of cars on the road, and now with a new injected 408 and 4 speed automatic and 3.91 gears, it's quicker and faster than most of them too.
It's the package.