I have Firestone Airbags and they nothing to do with the axle snubber or bump stops. Mine mount outside of the frame.
I believe Firestone makes a kit that mounts inboard of the frame for gooseneck/5th wheel hitches. The inboard kit might interfere with the bigger aftermarket exhaust systems. Might want to check with that first. I'm not that familiar with them.
I think the ones that mount between the axle and the bump stops are for the newer superduties. I'm pretty sure both kits for the obs (both with gooseneck and without gooseneck) mount on the outside of the frame. At least that's how it used to be.
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