Here in the printer’s manuscript, scribe 2 initially wrote “to behold of his glory”, but then he erased the of. Oliver Cowdery, when he proofed 𝓟 against 𝓞, restored the of. The 1874 RLDS edition also removed the of here, but it was restored to the RLDS text in the 1908 RLDS edition.
The of is unusual here, thus the tendency to remove it. Elsewhere in the text, we only have examples of “to behold something” (excluding, of course, the gerundive use of the verb in Moroni 10:14, which requires the of: “and again to another / the beholding of angels and ministering spirits”. Nor are there any examples of “to behold of something” in the King James Bible (excluding once more a gerundive use which requires the of, in Ecclesiastes 5:11: “and what good is there to the owners thereof / saving the beholding of them with their eyes”). Here in Alma 12:29, the critical text will maintain the unexpected original reading, “to behold of his glory”. Oliver Cowdery would not have supplied the of in 𝓟 unless 𝓞 read that way.
Summary: Retain the preposition of in Alma 12:29, the original reading (undoubtedly intended, despite its unusualness for modern English readers).