I just realized I wasn't quite correct on what I said. The 70-73 bikes used the 1/4 inch axle plates, the 74 up used the thicker plates. This is why most builders use the 1/4 plates. That way you can use the early rear wheels.
12 mm is just a hair under 3/8 inch and I said about 3/8 inch. I say 1/4 some may call it 6mm.
Maybe some builders use the thicker plates but from what most on here say they get the thinner plates.
Me, I don't think I would ever do that to a bike. The only bikes that should have hard tails are ones that came that way, as in the very old bikes.
Leo