Although I don't have any direct information on the 42nd Mississippi, I have researched the 2nd Mississippi of the same brigade extensively, including the Overland Campaign. My research shows the 2nd Mississippi mustered an aggregate present of approximately 280 men on May 1, 1864 just prior to the Wilderness. By July 1, 1864 the aggregate present was reduced to some 146 men, or a 48% loss rate (killed, wounded, missing and captured). Since the 42nd was involved in the all the same engagements during that period, I would imagine their loss rate was very similar. If you assume 20-25% of the total killed and wounded were actually KIA or mortally wounded and if you know the approximate strength of the regiment at the start of the campaign, that at least should give you some idea of the total number of fatalities you are looking for.

Hope this is of some help...

Mike