Will these wheels fit my 68 Barracuda?

I'd say you're right on the borderline, as KosmicKuda suggests. The "minus one" combo of 215-70-14 with 3.25" (factory) backspace fits no problem, on a 5.5" rim. It would run a little wider on a 7. And it's pretty well established that a 225-60-15 is no go without fender trimming on the leading edge.

How are you folks measuring the backspace? My understanding is that the nominal rim width is measured on the inside, where the bead sits -- if you measure from the outer rim it is 1/2 inch wider. So when you measure the back space from the mounting surface to the outer rim (you can't measure to the bead on that side), you need to subtract 1/4" -- right?

Just so we're all on the same page...

Tire Rack, Jegs, etc. have articles that say backspacing is measured from the wheel mounting surface to the outside edge of the rim, not the inside where the tire seats.
Measuring Backspacing - Turk | Tire Rack

Kosmic, I don't know what LBP means, unless it's something like "late-model brake package." Please help me out on that.

My '67 Cuda has front suspension/disks from a '74 Duster, the wheels/tires are 14x6 with 205/70-14's (everything like this when I got it). Haven't measured the backspace yet, but the tires rub the front subframe at full lock & simultaneously nearly touches the front wheel opening lower edge. I don't see how it's possible to go any wider in any direction, so I have no idea how y'all are putting such big wheels/tires on these Cudas, unless the wider track Kosmic mentions really hurts the situation.