This has been covered a thousand times... Set me on right course

Ok; just for looks , have you considered the much cheaper option of custom backspace wheels.

F-body rears will have a slightly wider track, and some come with the iso clamps, both of which require extra work to install.

Just for looks also means you can run adapters . But those also push the wheels out ~1 inch per side, so wide wheels will usually stick past the body line, usually requiring suspension mods to work..... and may not be legal to run,where you live.

If you have a slanty rear end in that 68, it will have the small tubes and 9inch brakes and small leafsprings. Whereas the big bolt-pattern rear will most likely have 3" tubes and 10 inch brakes, and wider springs. The point is you will need to swap everything between the spring mounts over, and probably re-engineer the brake bias.

And finally, the factory rear tubs on a 68 Barracuda will accept
275s on a 15x8.5 standard backspace wheel;with no narrowing, and no custom offset; or
295s on 15x10 wheels with the springs moved over just a lil. To pull this off will require custom offset wheels; or a narrowed rear end to run a 15x10 standard 4.5" backspace wheel. The 10s with 4.5bs wheels are a deepdish, and look really nice tucked in there.