Except for here in Jacob 5:8, Joseph Smith (in his editing for the 1837 edition) changed all the narrative examples of saith in the olive tree allegory to said (30 times). There is one other saith, which prefaces the allegory in verse 3 (“for behold—thus saith the Lord—I will liken thee O house of Israel like unto a tame olive tree”), but this saith is not, strictly speaking, a part of the narrative (that is, between the master of the vineyard and his servant) in the allegory. Apparently, Joseph missed editing the saith in Jacob 5:8 because the inverted word order is the same as in the familiar “thus saith the Lord”. For two other examples of where Joseph missed editing saith in a similar expression, see 1 Nephi 17:53 (“saith the Lord”) and 2 Nephi 6:11 (“for thus saith the angel”).
The critical text will, of course, restore all instances of the historical present that are found in the earliest textual sources, including here in the allegory of the olive tree. For one case where the historical present could be in error, see the discussion regarding calleth under Jacob 5:75. For a complete list of all the changes from the historical present to the past tense, see historical present in volume 3.
Summary: Restore each instance of the historical present tense saith throughout the allegory of the olive tree (Jacob 5); the original saith in Jacob 5:8 that has never been edited to said will, of course, be maintained as saith.