1971 Demon 17" Help

I crawled all over a 1969 Barracuda yesterday. I had been around them all my life but it was the first time I ever looked in the rear wheel housing and specifically looked at mini-tubbing possibilities.
The challenges with tubbing or a reverse tub is the trunk support in most cars. Not much of a problem with those cars but the problem is that there is a protrusion (bump) on the wheel side of the inner housing. I followed this bump to the inside of the car and it is the indention for the rear folding seat hinge. There is no wiggle room. If the metal wasn't there where the bump is you would see seat hinge IN the wheel well. You would have to section the back seat to retain it. It's impossible to take the inner tub to the frame rail without seriously reworking interior items that live between the wheel tubs.
I think the only way I would ever do one of those is if one wasn't so complete and just dedicate it as a race car and lose the entire factory interior.

Simply put, the rear seat hinge to hinge distance is about 2" wider than the rear frame rails they sit above.

First, my point had nothing to do with mini-tubbing. The stock wheel tubs on a Barracuda are the same as a Duster/Demon/Dart Sport. In fact, I believe that all '67-'76 A-bodies shared the same inner tubs, you can see my '71 Dart has the same indents as my '73 Duster and appear to be the same as the Barracuda (hard to say with the bracket in the way in the picture). The replacement inner tubs are all the same. The outer tubs are different, but shared by the Barracuda/Demon/Dart Sport/Duster. Those outers are 1” wider than the hardtop Darts/Scamps/valiants. So 275’s fit with no modifications, and 295’s will fit with a 1/2” spring offset and a little trimming on the quarter lip. A reverse tub like I did on my car can probably accommodate 315’s.

Here's a Barracuda, by @Bodyperson. It's already mini-tubbed here but the bracket is in the same spot on the tub (he narrowed the seat). You can kinda see the indents
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This is my '71 Dart. Same indents, just no brackets since a hardtop Dart was never offered with a fold down seat.
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Second, ‘73 up Dusters/Dart Sports with fold down seats have the same indent and similar bracket on the inner tub. Same exact issues as a Barracuda if you wanted to mini-tub. But it’s not like no one has mini-tubbed a fold down seat car, it has and can be done. And you can do it without cutting the seat down. You’d just remove the brackets, widen the tubs, and weld them back in.

Seat bracket on a factory fold down seat Duster
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You can see in the picture above of the Duster bracket, and in the picture of the Barracuda above, that the seat bracket actually locates the seat pretty much even with the forward part of the wheel tub. The large 90° bracket extends around the corner of the tub and gets welded into the indent. But the part where the seat actually bolts to the mount is less than a 1/2" from the front of the tub. The upper latch bracket also wraps around the tub, but again, the actual latch is well forward of the front of the tub. So, if you got rid of the wrap around sections and cut back the pass through panel

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You could basically just weld a flat plate with the threaded nuts attached to it in the same location as the original seat brackets. The bolts for the seat bracket would then extend into the wheel tub, so you'd have to seal them, but it's not like other bolts in the car don't already do this (seat belt mounting bolts, for example). You might even get away with a couple of large dimples in the wheel tub if you used a short bolt. The result would be that you don't have to section the rear seat at all, just deal with the small gap you'd get because the bottom of the fold down seat would move forward a smidge. On the later fold down Duster/Dart Sport cars it wouldn't require anything at all, there'd just be a slightly larger gap between the folded down seat and the pass through. The Barracuda might take a little more tweaking because I believe it does the pass through and fold down a little different.

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But certainly people have done mini-tubs on Barracudas, and it's really no harder than doing one on a fold down seat Duster/Dart Sport. There is still some work to do on the interior panels, but that would apply to both the Barracuda and the Duster. Bottom line is I wouldn't avoid a Barracuda because of the fold down seat. And if you don't mini-tub the Barracuda is probably the best car for the widest tires possible.