The 1858 Wright edition inexplicably replaced the modal verb shall with the fairly rare second person singular form wilst, as if the text read thou wilst when what we expect, if that were the case, would be thou wilt (the -st ending is due to second person singular forms like wouldst and shouldst). The form wilst is not found in the online Oxford English Dictionary, but I found a few examples of thou wilst on Literature Online (including one from Queen Elizabeth I). There are also quite a few instances of wilst on (accessed on 13 June 2005), but virtually all of these are in imitation of archaic English (such as “thou wilst be mine” and “thou wilst pass through lands of magic”). Not surprisingly, this bizarre 1858 reading was not copied into the RLDS text (the 1874 edition).
Summary: Maintain the modal verb shall in Mosiah 26:27.