I've seen original 14x6, 14x7, 15x6, 15x7, & 15x8 but never any 14x8. Not saying they were never made, but I've never seen nor heard of 14x8 rallies (or that size in any other style OEM wheel) in all of my years messing with and reading about 60's/70's Chevy muscle cars.
I realize one can order new rally wheels from various suppliers in a variety of sizes/offsets (including 14x8) but the ones advertised above are purported to be OEM, one year only, from the factory Rally wheels. If such an animal does exist, I would like to see some supporting evidence.