Firm Feel setup pics below. Some have said the bracket fit to the framerail is not so good. I have not used this setup. As said, you can drill out the factory spot-welds, install the new bushings (rubber or polygraphite) and plug weld up the holes OR you can install carriage bolts using the factory brackets.
First two pictures are the Firm Feel setup.
The last two pictures are my factory brackets with stainless steel carriage bolts.
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Yup: I have the FF setup for my 68 Charger. It fit well enough but rail profile varies a bit side to side so you may have to juggle things to get a fit that is both snug and level. It was impossible to get it exactly to where their instruction suggest a nominal location, but I got it very close.
The factory rear bar I put on my Challenger fit up without issue, but I had to adjust clearances around the P-brake cables and the 3" TTI exhaust.
If the OP's bushings are really shot, I'd see if FF would sell just the brackets. Their studs were pretty flat on the rail side.
Or, like you said, carriage bolts are a good solution, but you may have to grind the heads down to get a good fit.
Plug welding might roast the new bushings.