Here in the printer’s manuscript, Oliver Cowdery initially wrote “behold I do not say these things shall be of myself”; then later (with somewhat heavier ink flow) he supralinearly inserted now after behold and that after say. For both corrections, either the initial reading or the corrected reading is possible. Thus we can safely assume that the corrected text in 𝓟 represents the reading of the original manuscript, no longer extant here. Oliver probably made these corrections when he proofed 𝓟 against 𝓞.
The phrase “behold now” is fairly common in the Book of Mormon, with 11 other sentenceinitial occurrences in the earliest text. Of course, there are many more instances where the sentence begins with simply behold. For discussion of another case where the scribe (in this case, scribe 2 of 𝓟) accidentally omitted the now after a sentence-initial behold, see under 3 Nephi 20:10.
As far as the second correction goes, given the verb say and a following sentential complement, we normally find the subordinate conjunction that, but in some cases the that is missing; see, for instance, the discussion under 2 Nephi 30:2 or, more generally, under that in volume 3.
Summary: Accept Oliver Cowdery’s two supralinear insertions in 𝓟 for Helaman 7:29 (“behold now I do not say that these things shall be of myself”); since these two inserted words now and that are not required, the original manuscript probably had them.