The standard word tremendous reads as tremendious in the manuscripts (with various spellings). There is one other occurrence of this word in the text, and there too the manuscript reading has the extra /i/ vowel:
As explained under 1 Nephi 17:25, the earliest text of the Book of Mormon prefers grievious rather than the standard grievous. Similarly, we have tremendious rather than tremendous in the manuscripts. The critical text will restore this form of the word, perhaps the result of dialectal overlay but also quite possibly the original reading. This pronunciation was common in the 1800s, as exemplified by the following instances of tremendious from Meriwether Lewis and William Clark’s journals (1803–1806); the spellings are regularized except for the word tremendious:
The Oxford English Dictionary does not mention the form tremendious under tremendous; but under the word stupendious, the OED does refer to tremendious, identifying it as the “vulgar form of tremendous”. There is evidence, however, for tremendious as a more or less standard form in earlier English; Literature Online lists, for example, three formal uses of tremendious in poetry that date from the early 1700s (here given with original accidentals):
Thus the use of the word tremendious in the Book of Mormon is not necessarily dialectal but may represent a standard pronunciation of the word in Early Modern English. The critical text of the Book of Mormon will restore the original tremendious in all three instances.
Summary: Restore the original occurrences of tremendious in Alma 28:2–3 and Mormon 8:2, the consistent reading in the manuscripts for the standard tremendous.