You'll have to ask Steve, autoxcuda, for "proof", if it exists. The thread says the rims are 17x8's with 5.5" of backspace, so if the tires look a bit small its probably because they're a little squeezed on those rims, 275's should be on a 9" rim.
If all goes well, I'll have a set of 275/35/18's on the front of my '74 Duster next week though. I know, not a barracuda. But I'll be able to provide measurements firsthand at that point.