If you think the .508 head dia is .050 too large you may have read the wrong line for the .40-70 Sharps Straight which would be about .453.
The 40-70 Sharps BN used the .503 45-70 basic head size with the 2.25 case length.
You might also note at one time there was a wildcat round called the .40-70 Govt that was just the 40-70 BN chamber with a 2.1" long case from the 45-70. I have not heard of the .40-70 Govt in years and I have never seen a set of dies so standard .40-70 BN dies may have been used.