ANY AUTOCROSS RACERS
You can do pretty well with a set of zero arch style leaf springs and a 120 to 130 lb/in spring rate. You can also replace the rear shackles with a set of leaf spring sliders, which will reduce flex by ditching the shackles and bushings and improve spring reaction times (less resistance than shackles). That and a good, adjustable rear sway bar. Peter Bergman spec’d a great set of springs from Eaton, there’s the hotchkis leafs, and even the Mopar oval track springs that were recently reissued if you’re on a budget. Not fancy but they’re zero arch and the rate is in the ball park. I'm not sure what Firm Feel is selling now, they used to sell the Mopar Oval track springs but based on their description it seems like they're doing something custom now.
I know @autoxcuda runs the Hotchkis leaf springs on his Barracuda, so they should work for your fastback too. I've heard both that they work great and that they don't work on the 111" wheelbase cars, but I've seen them on those too. Pretty sure that's what @MuuMuu101 is running on his Dart. But Peter has a lot of time on the Eaton springs he has on his Dart, so I wouldn't discount those either.
I have my Duster set up with 121 lb AFCO's, which are no longer available. Pretty sure that's a licensing thing with the Mopar Oval Tracks, because the AFCO's in that spec went away at the exact same time the Mopar Oval Tracks came back. The oval tracks I bought for my Dart also look pretty much identical to my AFCO's, but I haven't mounted them up yet. I think the AFCO's work really well, but that's just based on street driving so far. I run them with spring sliders and a 7/8" rear sway bar.
My slider set up
Resulting ride height
You can do pretty well with a set of zero arch style leaf springs and a 120 to 130 lb/in spring rate. You can also replace the rear shackles with a set of leaf spring sliders, which will reduce flex by ditching the shackles and bushings and improve spring reaction times (less resistance than shackles). That and a good, adjustable rear sway bar. Peter Bergman spec’d a great set of springs from Eaton, there’s the hotchkis leafs, and even the Mopar oval track springs that were recently reissued if you’re on a budget. Not fancy but they’re zero arch and the rate is in the ball park. I'm not sure what Firm Feel is selling now, they used to sell the Mopar Oval track springs but based on their description it seems like they're doing something custom now.
I know @autoxcuda runs the Hotchkis leaf springs on his Barracuda, so they should work for your fastback too. I've heard both that they work great and that they don't work on the 111" wheelbase cars, but I've seen them on those too. Pretty sure that's what @MuuMuu101 is running on his Dart. But Peter has a lot of time on the Eaton springs he has on his Dart, so I wouldn't discount those either.
I have my Duster set up with 121 lb AFCO's, which are no longer available. Pretty sure that's a licensing thing with the Mopar Oval Tracks, because the AFCO's in that spec went away at the exact same time the Mopar Oval Tracks came back. The oval tracks I bought for my Dart also look pretty much identical to my AFCO's, but I haven't mounted them up yet. I think the AFCO's work really well, but that's just based on street driving so far. I run them with spring sliders and a 7/8" rear sway bar.
My slider set up
Resulting ride height