In the first printing of the 1852 LDS edition, the preposition of was deleted here, probably accidentally since it was restored in the second printing of that edition. Nonetheless, the of does seem intrusive. Of course, the “yea exceeding” itself is interruptive, breaking up the clause “the beauty thereof was far beyond all beauty”, in which no of would have occurred. In this instance, the word exceeding is verbal and the of makes it a gerund, so there is really nothing wrong with this construction, even though its usage is unique within the Book of Mormon text. The of was obviously intended.
A similar example of such an interruption involving beyond uses the conjunction or:
In this instance, the beyond is necessarily repeated.
Summary: Maintain the preposition of in 1 Nephi 11:8 (“exceeding of all beauty”).