I think you're having a tough time selling it cause of the black 67 stripe, color change, original paint not upper end desirable, and aftermarket engine stuff. That narrows the buying audience. Posting built sheet and fender tag with decodes would help since it's missing "S" badging and other emblems.
I don't think sub frame connectors will hurt. They are not the end of the world to remove. But I wouldn't go with the ones that you continuously weld down the floor boards.
You can fit 275/60/15 on that without mini tubbing and right backspacing. Mini Tubing would extremely narrow the buying audience.
Are you changing the motor or trans when you take it to the next level. What are your plans?
But getting a real clean southwest 68 Barracuda 318 in the same condition as yours would probably be like $10K by the time it's delivered to your door. Maybe more. IMHO, about $5-6K less than what it would sell for. You probably have than that much in parts you'd buy for a donor that are already on your car.
And if you are happy with paint on your car, you couldn't get a non S car as solid and put paint on it for what your car would sell for.