It really isn’t much. I run an 18x9 +35 on the front, it basically positions the wheel the same as a +30.
And if you’re willing to do some fender work Craig has shown you can go all the way to +12, which he’s made look a lot easier than I ever thought it would be. There’s a lot of rims in the 18x9.5” size with offsets in the +20 to +25 range. I know because I’ve looked at them but discounted them because of the offset. With your Barracuda and the work you’ve already done on your fenders I don’t they they’d be that much work with the right tires.