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...