One wonders here if the change of unto to into (only in the 1841 British edition) might be correct, even though the original manuscript is quite clear in reading unto. It is possible that Oliver Cowdery, the scribe in 𝓞, mistakenly wrote unto instead of into, just as he often did in the manuscripts (for a list of examples, see under 2 Nephi 8:23), including the following one that occurs in a phrase referring to a land:
This change provides some evidence for a tendency to incorrectly write unto in place of into in the context of land.
Elsewhere in the text there are 17 examples of “go into a land”. The only example with unto instead of into is here in Helaman 3:3. But when we consider other verbs besides go, we have additional evidence for an occasional unto. For example, when the verb is come, there are 27 examples with “into a land” but also 4 of “unto a land”:
Since come and go are both very similar semantically (differing in directionality), it seems appropriate to accept the possibility of “and went forth unto the land northward” in Helaman 3:3.
Summary: Accept in Helaman 3:3 the reading of the original manuscript, “an exceeding great many … went forth unto the land northward”.