As discussed under Alma 50:2, the earliest text had heighth instead of the standard height. Here in Helaman 14:23, 𝓞 most likely read heighth, which the 1830 compositor emended to height when he set the text from 𝓞.
This passage also shows a vacuous use of the word thereof. As discussed under Alma 46:12, there are a number of places in the original text where thereof appears to have no referent. Such instances were, for the most part, removed from the LDS text in the 1920 edition. The critical text will restore them.
Summary: Restore the original heighth and thereof in Helaman 14:23: “which shall become mountains whose heighth thereof is great”.