The original text here reads thou saidst, with the standard -st ending for the past-tense said when the subject is thou. This reading was changed to thou said in the 1837 edition, which continued in the editions until the 1870s. The 1874 RLDS edition corrected thou said by adding the perfect auxiliary have, giving thou hast said (but ultimately, in 1908, the RLDS text restored the original thou saidst by reference to the printer’s manuscript). In the 1879 LDS edition, the correct thou saidst was restored to the LDS text. Orson Pratt, the editor for that edition, may have consulted the 1830 edition, but that would not have been necessary since thou said is grammatically nonstandard. In any event, the current texts, both LDS and RLDS, maintain the original reading, thou saidst.
In most cases, the earliest text has the -(e)st ending for the simple past-tense verb form when the subject is thou, but there are a few cases where the -(e)st is lacking. In the following list, I first give one example of the standard usage, then every case where the -(e)st is missing (marked with an asterisk), whether in the earliest textual source or secondarily in an edition:
thou didst (18 times) versus thou did (2 times)
thou said(e)st (2 times) versus thou said (0 times)
thou hadst (1 time) versus thou had (1 time)
thou comforted(e)st (1 time) versus thou comforted (0 times)
thou receivedst (0 times) versus thou received (1 time)
In the earliest text, then, there are four instances with no inflectional ending for the simple pasttense verb form when the subject is thou. And there is one case, here in Helaman 11:14, where that ending was omitted in one of the later editions. Ultimately, the LDS text has none of these nonstandard forms except for the one in 2 Nephi 24:12 where the thou is in an earlier clause. The critical text will restore these few instances where the earliest extant text lacks the -(e)st ending. For further discussion, see under 2 Nephi 24:12 for the use of did in the relative clause “art thou cut down to the ground which did weaken the nations”.
Summary: Maintain in Helaman 11:14 the standard thou saidst, in this case the reading of the earliest textual sources.